Your question in itself identifies one of the key issues in the historiography of the Civil War. What was the point of it and when and how did attitudes begin to shift between one which emphasised the end of slavery alongside the preservation of the Union. Ultimately this is a question which cannot be answered with certainty -- that on a specific date it shifted from one to the other. What we are left with is to say that the transition took place over a relatively long period, but that in retrospect both historians and contemporary actors recognised that it had taken place.

So, let's go back to the beginning and the origin of the idea of a war for the Union. The idea has its origins, as you might imagine, in the constitutional crisis which led to secession. Most in the North, for their part, argued and felt passionately that the union created by the constitution was eternal, that the states gave up their sovereignty to the federal union and that therefore there was no constitutional right to secede. This, for instance, was the position that Lincoln articulated strongly in his first inaugural address in 1861. He was then echoed in much of the Northern Press. The St Paul Daily Minnesotian commented that "the question comes down to one of two things, resistance or submission. We have chosen resistance to the encroachment of slave power" and continued that "as we have supported those principles with out voices and our votes, let us, if necessary support them with our arms and hands." The Boston Daily Atlas and Bee in early February 1861 commented for its part that "it is generally agreed that the individuals who are concerned in the demonstrations consequent to the secession movement are guilty of treason... the issue is not disunion or concession, it is the triumph of treason or the triumph of law." As such, we can see that many in the North felt that there was a legal basis to defend the Union with arms. Indeed, for many Americans the idea of rallying around the constitution and the flag against a group of oligarchic traitors was an attractive one. Indeed, the bombing of Fort Sumter led directly to spontaneous rallies and protests across the North, as was vividly described in the diaries of George Templeton Strong. It's hard to get into the heads of the Union populace to understand why they fought, just as we might question why people fight for nations in the first place. However, many had well developed understandings for why they fought. Fred Spooner, a lieutenant, wrote to his father in early 1861 that "traitors have begun the conflict. Let us continue it and end it. Let us settle it now, once and for all... they have no chance to overturn this government. They haven’t the resources... they have prospered dealing in human flesh – let them now take the results of it." A merchant called Samuel Hinckley wrote to his son that "you can have little idea of the actual excitement and unanimity of all parties in the north, east and west in putting down forcibly the gigantic southern rebellion... the clouds are lifting and all looks brighter for the safety of our country and the perpetuation of the union"

There was the option from the outset to make the war about the issue of slavery. However, the government shrunk from the idea. Seward for instance wrote to Lincoln forcefully before the attack on Fort Sumter arguing that "CHANGE THE QUESTION BEFORE THE PUBLIC FROM ONE UPON SLAVERY OR ABOUT SLAVERY for a question upon UNION OR DISUNION." While there were many ardent abolitionists in the North at the time, they were a small minority. Most Northern Americans were, for the most part, just as convinced of the idea of black inferiority as their Southern brethren. Lincoln and others might well be included in this category. As such, it made little sense for the administration to emphasise what might be viewed as an illegal seizure of property and an encroachment on the rights of the states. Any such move would have inflamed Northern opinion to such an extent that prosecuting the war would have been impossible. Further, Lincoln desperately required the cooperation of the border states of Maryland, Kentucky, Tennessee and Missouri. Continued toleration of slavery there was an essential pre-requisite to keeping them in the Union fold. In Lincoln's own words, "I think that to lose Kentucky is to lose the whole game."

We should note that this Union only stance of the early years of the war was very unpopular with some and there was consistent pressure on the administration to change the meaning and aims of the war. In press examples of this include Greely's "Prayer of the 20 millions" of 1862 which begged Lincoln to destroy slavery. Other examples include actions by his commanders which threatened to turn into an outright attack on slavery, such as Fremont's proclamation of emancipation in Missouri in August 1861 and Hunters similar declaration in May 1862. In both instances Lincoln rescinded the orders immediately and reserved the right to change the course of the war to himself and not his subordinate commanders. Outside of the US it met with a certain amount of bafflement and confusion. The result was an active press campaign by Union propagandists like J.S. Mill and John Bright to advocate for the Union and explain why respectable British opinion should support the Union, even when there was no clear moral case for them.

So, how do we get from there, with a seemingly single minded pursuit of the Union, to a self conscious war for slavery and the Union together. In my view there are two parallel stories here. The first is a change in Lincoln's own views on the issue and the second is the facts on the ground. For Lincoln, he seems to have undergone a personal journey over slavery and began to view its extinction as the only way to win the war. In his own words, he viewed emancipation as a "lever" to save the Union. If he could have the Union without emancipation, he would take it but since it seemed that he needed emancipation that was fine as well. This was a policy that had been in the works for some years. Union officers, congressmen and the general public recognised that former slaves could be used as weapons against their former owners. This can be seen in the first and second Confiscation Acts of August 1861 and July 1862 respectively. The second act was particularly important because it allowed for the confiscation of all Confederate property and the transfer of all captured slaves to the Union army.

As you said, your history teacher said that actual experience of the plantation system had something to do with changes in attitude to slavery. Their frequent correspondence home then disseminated information throughout the northern populace. I don't want to discount this but rather to nuance it slightly -- there were hundreds, and possibly thousands of accounts of widely read reports on the brutality of plantation life in the south. The northern public, in other words, had abundant information about way that society operated in the South before the war and that does not seem to have changed much. The writings of abolitionists like Douglas and Garisson do not seem to have moved the wheel much. Similarly, less 'biased' accounts like those of Frederick Law Olmsted and Hinton Rowan Helper do not seem to have changed much either. Instead, I believe that a mix of increasing abolitionist sentiment, combined with a powerful public relations campaign from people like Garisson, a pragmatic desire to end the war and use black labour and soldiers (one letter from the governor of Iowa to General Halleck comments that “I shall not have any regrets if it is found that a part of the dead are niggers and that all are not white men”) and a vindictive desire to punish the South led to a complete sea change in Northern opinion on the issue of slavery and led to the twin elevation of the Union and slavery. The result is shocking for us even in the present day and was just as astonishing for people at the time. The New York Times, for instance, commented "it is extraordinary how completely the idea of gradual emancipation has been dissipated from the public mind.” Gradual emancipation, which in 1859 had been little more than an abolitionist's fever dream was, by 1864 far too conservative a policy. The Chicago Tribune, in the aftermath of Lincoln's election went even further: “the people demand the destruction of slavery as the deadliest foe to the country and the real life and support of the rebellion... we can have no permanent union and no enduring peace until it is destroyed” and judged that “the election is simply the formal utterance of that conviction.”

There are several good documentary collections of Civil War Soldiers. One of the best is "Yankee correspondence : Civil War letters between New England soldiers and the home front" edited by Silber and Sievens