This isn't the same old "was slavery the cause of the war" or "what was the civil war about" or "did the north go to war to stop slavery" question.
I'm literally wondering the answer to this question: why were the people of the north against the expansion of slavery prior to the civil war?
Today we (mostly) know and accept how horrific slavery is. However, I'm not about to take the step and say the people of the north were somehow morally superior to the people of the south and thought slavery was horrific, so it needed to go.
Nowadays you'd poll everyone: Are you against slavery? Which of the following reasons is the primary reason? Obviously this didn't happen back then.
So, was it about economics - the northern fear that the expansion of slavery would continue to put them on unequal footing with the south, who got low priced labor? Otherwise, considering racism in the north by the majority of people (as far as I can discern), the acceptance by the majority of the idea that black people are genetically inferior, etc, doesn't lead me to believe it really was some great moral crusade by the majority.
Obviously, different people have different reasons so there isn't one answer for every citizen of the north, but what was the most common reason?
And has anyone written about this or studied this in detail? Any books about just this question?
There were various reasons for the North being opposed to the expansion of slavery, and its convenient that you asked this today because I'm in the middle of The American Civil War: A Hands on History by Christopher J. Olsen and he has a great chapter on this exact topic.
Many Americans in the 1800s, both north and south, were religious. Both sides used religion to aid their cause. Slavery proponents preached that slavery was a great way to introduce slaves to civilization (read: Christianity). Anti-slavery people, including many northerners (but not all!), felt that slavery was a horrible sin against God, holding it in equal standing with things such as murder and rape. In the book, Olsen writes, "Slavery was likened to murder, and no ones encourages murder to 'just cut back a little' this year; you also don't pay murderers to stop." Basically, many northerners felt that slavery was a great sin against God and would lead to the damnation of America. That being said, many not only wanted it not to spread - they wanted it to end completely.
Politics represented another reason that northerners did not want the spread of slavery. As mentioned by /u/jad4400, the north and south had separate, distinct cultures; the southern economy was based primarily on slave labor, while the north used a competitive labor market. As a result, the two sides had starkly different interests. Before the war, the north and south competed to control the majority of Congress and thus, run the government. Due to their different interests, the two typically wanted to pass different legislation or enact different policies, each side wanting to help their own cause. If slavery spread or was permitted in newly admitted states, that meant that the pro-slavery south would gain a few more seats in Congress, thus giving them more political clout. New states basically represented new seats in Congress - the North and South both wanted these seats. So, the north didn't want slavery to spread because they wanted to have the majority in the government. Conversely, the south wanted the spread of slavery because they wanted more power in the Congress so that they could promote their cause. For this reason, the south was outraged when California was admitted as a free state. The southern feeling of powerlessness within the government, signified by the election of Abraham Lincoln (he wasn't even on the ballot in 10 southern states) would eventually aid the decision to secede.
Finally, we often have this notion that the North was made up of righteous, morally sound people. This is not necessarily true; most northerners would be considered racist by our modern-day standards. The majority of northerners, including Abraham Lincoln, felt that blacks were inferior to whites. As a matter of fact, Lincoln once said in a speech that, "Certainly the Negro is not our equal in color - perhaps not in many other respects." This sentiment led many to not want to live in the presence of black people. The spread of slavery to the North would significantly increase the black presence. Essentially, many white northerners thought that black people would tarnish society, ruining the (almost) perfectly white society that they currently had. An example of this can be seen when Oregon became a state. When admitted Oregon not only prohibited slavery; they banned black people from living there completely (shocking, right?). So essentially, many detested the spread of slavery out of sheer racism; they simply didn't want black people around.
As you said, there were probably countless reasons for the opposition of slavery and its spread. For more information, I do recommend The American Civil War: A Hands On History. It discusses this topic in more detail and gives ample information through primary sources without being too dense.
I also took some of this information from Ken Burns' The Civil War. Great watch if you ever have 11 spare hours!
I hope this helps to answer your questions! Sorry its so long!
EDIT: Forgot one more quick thing. Many northerners working what would be considered lower-level jobs feared that if slavery expanded into the North, they would lose their jobs to slaves.