Your teacher is wrong. Aside from being factually wrong, they are morally wrong, peddling what is nothing more than outright apologia for American slavery that has been peddled for centuries, and originated within the community of enslavers seeking to justify their ownership of another human being both to themselves and to others.

Now, to be clear, some enslavers were, by objective measures, less cruel than others. There is no disputing that, although it is hardly a mark in the institutions favor. Those forced to slog through Uncle Tom's Cabin in high school may recall how the uncertainty of ones' position, Tom for a time having a comparatively nice existence which then was pulled out from under him due to forces outside his control being one of the many evils described by the abolitionist tract. In point of fact that, to focus on that misses the point entirely of what slavery is, which is what the historian Orlando Patterson terms "social death". It is not just the loss of power, but the loss of self. Slave systems throughout history have varied greatly, but the status of the slave as a socially dead person is the most consistent part, as well as that makes any claim of "nice" or "family" ring hollow.

I'm going to shift gears here and move centuries and an ocean away for the example that perhaps best illustrates this. In the Ottoman Empire, eslavery looked quite different from the chattel slavery of America, and slaves could, at first appearance, gain great status and power, such as Ibrahim Pasha, who rose to be the Grand Vizer of the Sultan, one of the highest positions in the entire empire. But he was still a slave, and still deprived of his own power, and his own sense of self. This is best demonstrated by an incident in his life where he had witnessed the commission of a crime, but at the trial, when he showed up to testify, he was prohibited from doing so. As a slave, he had no self, and no innate honor, so despite his lofty position, his words had no validity in the court, only when they were spoken on behalf of the Sultan. He was nothing more than an instrument for someone else to exercise power, a literal extension of the Sultan's person, but entirely unable to exercise his own independent of that role. In his own role, he simply had to stood there and be humiliated by a minor court functionary who denied him from testifying, and then hear the Sultan endorse the decision, a cruel reminder that he was in fact still the lowest of the low.

Now, as I said, this is wildly different from chattel slavery, but I use it to undercut what "nice" means here. When your teacher says that slave owners were "nice", he means that they used the whip less, or perhaps that when they did feel it necessary, they allowed the wounds to be dressed immediately. What he doesn't mean is that they recognized their enslaved persons as full human beings. The mere fact that they owned another person at all gives that the lie, as in doing so they inherently participated in a system that was perpetuated the dehumanization of their human property.

And to be sure, the slaves themselves were aware of this, even if the enslavers might be oblivious. In the recollections of his period of enslavement, Frederick Douglass remembered how his owner would occasionally give him a penny or two (I've written more here on slaves and property), but as Douglass relates:

I always felt worse for having received any thing; for I feared that the giving me a few cents would ease his conscience, and make him feel himself to be a pretty honourable sort of robber.

That is to say, from the point of view of the enslaved, the enslaver 'being nice' wasn't simply "being nice". It was also a reminder of their condition. This goes far beyond things like handing out a coin, but also gets to the core issue of punishments. When someone speaks about a slave owner being "nice", they mean, as I noted above, that punishment was rare or administered lightly. This doesn't mean punishment never happened.

One infamous example I would use is that of Robert E. Lee. Although the popular image of him is that of the conflicted, but honorable, Southern gentleman who held a personal dislike for slavery, this is a fairly erroneous picture in a number of ways, but he is generally held up as a "nice" slave owner, which again, is an oxymoron. What I would focus on here specifically is his use of punishment though, specifically when to of the people that his family owned tried to escape and gain their freedom but were captured and brought back. He certainly didn't hold back on a whipping for either of them, and he supposedly ordered that the wounds be doused in salt-water afterwards as well for an additional burst of pain. Even if we talk only in comparative terms, and state that as far as slave owners go Lee was hardly the worst of them, that is small consolation to the two men who wanted only freedom, and were cruelly punished in their attempt to gain it. A slaveowner being "nice" only went so far as the enslaved persons accepted their place as a slave. A master may not have had to use the whip much, but that didn't change the fact that the enslaved persons were still enslaved, and that their status was held up by the inherent violence of the system.

And even the mere threat was often enough, the potential for violence omnipresent reminder of their condition. One of the most common refrains you find about plantations where the master was "nice" was that the rarity of actual violence was undergirded still by the threat of it, or even worse, by the promise of being sold somewhere else: not only a place where punishment was much worse, but also breaking up families. This was especially the case in the Upper South, where it was generally true that slaves were less brutalized, and numerous recollections of enslaved persons mention the threat of "being sold down river" as punishment for disobedience, rumors of the plantations in the Deep South, and the severity of punishment one could expect there, used to keep them in line. This was hardly an empty threat either, and hardly alien to the "nice" enslavers, who even if they didn't have a stomach for harsh punishment were hardly averse to essentially outsourcing it.

Now, I'm mainly focused on what is meant by "nice" here, but I also want to focus on what is meant by "family" here. Much of the above plays in here too. The most obvious, and cutting response I would make is that I consider my dogs to be family, but that doesn't mean I consider them to be my equals, let alone human, no matter how lovable they are. As already discussed above, slavery was inherently dehumanizing, and a denial of the enslaved person's self, and that is important here. Many accounts of escaped slaves which are coming from the primary source documents written by the enslavers make mention of a sense of betrayal. They are hurt by the fact their property didn't want to be their property any more. When we talk about the enslavers considering their slaves part of the "family", this is what is meant. It isn't that they held real affection for them as an equal; it isn't that they understood them as a person, it is, again, that they expected the enslaved person to accept their lot in life and conform to white expectations as a member of the household on white terms without regards for what the enslaved person might actual feel.

The idea of "the faithful slave" is an endemic one in Southern literature of the antebellum period, and of apologia since then, and this idea of treating them like family is a central component of it. Aside from entirely missing the inherent power imbalance and denial of basic humanity of the enslaved persons, it also misses the destruction wrought on the real families of the enslaved persons by slavery, thousands upon thousands of them broken up by selling part of a family group elsewhere. Treating their slaves "like family" had nothing to do with the real affection you hold for your partner, for your children, for your siblings, for your parents... It was about casting the enslaved person in the idealized image of loyal, faithful servant who knew their place at the bottom of the household hierarchy, and was defined entirely in their relation to white society, with no concern for the black society of which they were actually a part.

So in short, I would repeat what I said at the start. Your teacher is wrong. Very wrong. Slavery is inherently the dehumanization of another person, the consignment of them to social death. Although I've spent a good amount of time on concrete examples which give lie to the statement and lay out how claims about kind slave owners who thought of their property as family are nothing more than apologia written from the perspective of the oppressor looking to defend the institution and/or make themselves feel better about it, that is the philosophical core. Strip down everything else, and get to the root of what slavery is, and you arrive at the denial of self by the enslaver (I'd also point you to this answer of mine about slaves own attempts to reassert their sense of self). Whatever treatment might look like on the surface, however nice the veneer of it may be, that essential relationship can't be avoided, and absolutely invalidates any deeper meaning such superficial actions might otherwise imply. Slavery is one person or group denying the personhood of another, and I can think of few deeper cruelties.

Edit for a disclaimer: I assumed your teacher was referring to slavery in the United States. If I'm wrong about this, please let me know!

Short version: Your teacher is not only wrong, the telling of this narrative is a very specific strategy employed before, during, and after the US Civil War to justify slavery, to infantilize Black people, and to support systematic racial discrimination.

Your history teacher, in saying that slaves were happy and treated kindly, is perpetuating an idea that was used to justify slavery when it still existed and, later, to support an ideology referred to as the Lost Cause (title taken from an 1866 book), which argues that the US Civil War was not fought over slavery at all, and that slaves were happy and well-treated before the war. People who argue this frequently contrast this "happiness" with unemployment, alcoholism, violence, and other ways Black people suffered after the end of the war, while leaving out the role of institutions put in place specifically to create these effects, such as Jim Crow laws. Those who support this narrative say that Black people are by nature suited to hard physical work and that they are most content when doing it, but they need white people to "guide" them in doing it, or else they become idle and criminal. This ideology explicitly equates Black people with farm animals, bred and born to perpetual servitude, thanking white people who "give" them work. People who support this ideology might point to the idea that, after the importation of slaves was outlawed in the USA in 1808, slaveowners treated slaves better. They leave out, however, that this only meant slaveowners were more careful not to kill their slaves, knowing it would be harder to come by others. Torture and abuse continued; they were an inherent part of the slave system.

In what is known as the "plantation tradition," the plantation is presented as an idealized family, in which the white male patriarch of the plantation serves as a guiding father figure for the slaves, who are represented as perpetual children mentally who need strict discipline and someone to give their lives purpose through hard work. There's lots of evidence for this in popular culture, from the cheerfully maternal slave "Mammy" in Gone with the Wind, whose only desire is the happiness of the white people she is a slave to, to Uncle Remus in the Disney movie Song of the South, which was banned for its perpetuation of this myth of the happy plantation (it takes place in the Reconstruction era, so Uncle Remus is technically not a slave). Uncle Remus happily sings and tells stories to soothe and help a white boy whose family owns the plantation. In a move I think speaks volumes, dialogue in the script added by liberal writer Maurice Rapf that emphasizes that Uncle Remus is a free man and that the Black characters aren't slaves and must be paid was removed.

Here is a letter to The Confederate Veteran in 1893, encouraging the editor to include more narratives about happy slaves. The writer says he was a boy when slavery was legal, and finds it disturbing that people he loves could have supported something that sounds so horrible in all the reports about it, so he wants to read stories that prove to him that it wasn't so bad, so that he can feel better about it:

Another thing, our opponents have published tons of literature giving the dark side of slavery. We have little telling of its bright side. Although I was born during slavery times and was old enough before its abolishment to appreciate its existence, I have seen so much against it that the very idea of it is abhorrent to me. I have read so much of the dark side that I wonder how those whom I love so dearly could have upheld such an institution. If I am so influenced, what must be the feelings of my children when they grow up? Let each issue of your paper contain something telling of the bright side, of the corn shuckings, the quiltings, the barbecues, the big meetings, the weddings, &c., showing that the slaves enjoyed life and were not eternally skulking in dark corners dodging the whip of the brutal overseer, or quaking with terror at the bay of a blood hound.

One of the most frequently cited pieces of evidence that slaves were happy is the songs sung by "happy" slaves, which is why Frederick Douglass in his 1845 book Narrative of the Life of Frederick Douglass, an American Slave, and Written by Himself specifically writes about the songs slaves sang and their seeming joy when they were told to serve in the "great house" of the white plantation-owning family. (The idea of the "great house" concides with the ideology that an upper-class white family, as purveyors of civilization, class, propriety, and good taste, was ideally suited to giving order and value to the lives of slaves.) The songs seem to be happy and praising the "greatness" of the house:

They regarded it as evidence of great confidence reposed in them by their overseers; and it was on this account, as well as a constant desire to be out of the field from under the driver's lash, that they esteemed it a high privilege, one worth careful living for.

But let's not overlook Douglass's almost casual reference to being whipped all the time. He says the songs actually reveal the opposite of happiness: the desperation of slaves who face daily abuse and see getting away from the overseer and the lash for a short time as a small and pathetic respite from the worst abuses of the plantation. In Douglass's words:

I have often been utterly astonished, since I came to the north, to find persons who could speak of the singing, among slaves, as evidence of their contentment and happiness. It is impossible to conceive of a greater mistake. Slaves sing most when they are most unhappy. The songs of the slave represent the sorrows of his heart; and he is relieved by them, only as an aching heart is relieved by its tears. At least, such is my experience.

As further evidence against the idea of "happy" slaves, Elizabeth Keckley, also a former slave, writes about an incident of a mother having her child taken from her and sold. When she is found grieving for her stolen child, the mother is brutally punished. Keckley's account is revealing because she shows that the master insisted on slaves acting happy, and tortured them if they did not. The idea was that any show of unhappiness could encourage resistance, which might lead to refusal to work or rebellion. This is how Keckley describes it in Behind the Scenes, or, Thirty Years a Slave, and Four Years in the White House:

One day she was whipped for grieving for her lost boy. Colonel Burwell never liked to see one of his slaves wear a sorrowful face, and those who offended in this particular way were always punished. Alas! the sunny face of the slave is not always an indication of sunshine in the heart.

Keckley was seven years old when she witnessed this. This gives a lot of context to the behavior or even testimony of slaves who said they were happy with their treatment. Slaves were kept in purposeful ignorance and forbidden from learning to read and write. Some states made educating slaves illegal. Slaves were regularly lectured on their "proper" place and tortured. The law Partus sequitur ventrem, first established under the English and incorporated into US law, made it so children were born into the status of their mothers: this was specifically designed to allow white men to rape Black women, the resulting children being born slaves and having no right to the status or property of their fathers.

But what about stories from former slaves who described the plantation in positive terms? From 1936 to 1939, the federal Works Progress Administration perfomed the largest gathering of slave narratives, the Ex-Slave Project. Many of these narratives recall the plantation positively, and have formed a large part of the retellings of the plantation narrative. Articles and books, however, point out the deeply problemative nature of these interviews. In an era shaped by Jim Crow and the Great Depression, the relationship between Black interviewees and (in most cases) white interviewers was defined by inequalities and internalized racial roles. Some interviewers were members of the United Daughters of the Confederacy, whose stated purpose was perpetuating of the Lost Cause myth, putting up monuments to the "glorious" Southern dead, and putting Lost Cause textbooks into schools. Others included writer and anthropologist Zora Neale Hurston, whose interviews provide a stark contrast to those by white interviewers, showing how much difference the interviewer makes. The author of the book I linked to does a great job describing the strategies, often taken from Black vernacular tradition, used by interviewees to reinscribe their own stories and commodify themselves and their own stories in a framework defined by an inherently unequal relationship with their interviewers and the people running the project.

This is a huge and important subject. I was also taught in school the same thing you describe: that masters were kind to slaves and treated well. After all, my teacher said, you wouldn't starve, beat, or otherwise abuse your horse, would you? You'd make sure the horse was well fed, housed, and cared for. This comparison, however, is deeply dehumanizing. It reveals how people like my teacher think about Black people, and it is completely incorrect. Slavery at its core relied on torture and abuse whose purpose was to "break" people and force them to obey, and the portrayal of happiness about the plantation comes from a deeply unequal relationship.

This is just wrong, the overwhelming majority of slave owners were not nice to their slaves and treated them as animals and property. Very few if any saw them as family or treated them nicely. Historical revisionism and fiction by your teacher