The government today and the ideas of ordinary Americans are so different today from before the war that it is very difficult for us to even relate to it.
One simple but telling example is that before the Civil War no one ever said that the United States "is", the term was always the United States "are". Obviously, before the war, it was recognized that it wasn't a giant centralized stated, but a collection of equal states.
Before the war, political parties were much less important than the states. The states and their interests formed voting blocks much more than the parties did. Sure, parties had platforms, but those parties were usually regionalized and sectionalized. Today, a Texas Democrat is apt to vote the same as a New York Democrat because of loyalty to party and because they were whipped by that party to vote a certain way. In those days, that wasn't the case. Or rather, it wouldn't have happened that way because a guy from New York and one from Texas were unlikely to share the same party platform because of where they were from.
Thus, slave verses non-slave became the predominant way to separate the voting blocks in the country. So, adding slave or non-slave states without adding one from the other side, altered the balance of power. Once the South figured that slavery was to be excluded from the territories, they understood that they were to be out voted on everything. There was little fear that slavery would be immediately abolished, but like gun owners today, southerners could see the future. When the Democrats of today get an unassailable majority because of demographics, we will have one party rule. Gun control and many other things will become a reality because they will control all the branches of government. There will be no check on their power. The South simply decided to get out of that losing bargain while they still had power to have a fighting chance.
And if you want proof that the war was about much more than slavery, just look at voting patterns today. The South still votes almost uniformly as a block as does New England. And they are almost always on opposite sides of the issue, whatever it is.
National unity has ALWAYS been a myth imposed at the muzzle of a gun.
