Divergent economic situations were a big driver, in part due to the slavery plantation system. The north industrialized and grew both the economy and population (immigration) faster than the cotton slave plantation economy could. The northern industries benefited from tariffs while tariffs stifled cotton exports through reciprocal tariffs from trading partners. The slave based cotton economy was bottled up in southern states and couldn't expand westward. Mining and building railroads could have been done by slave labor instead of immigrants and the southern elite saw laws against westward expansion as an unfair cap on their own economic growth. On the other hand, the more populous north wanted those railroad and mining jobs to go to their burgeoning population. And the north had the money and numbers to make legislation go their way. Without the ability to expand into the west, the slave states were capped to an agricultural economy. And tariffs hurt the south while they helped the north.
http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/civil-war-economic-causes-issue
So, yeah, slavery was the foundation on which conditions that led to war grew to crisis levels. But it wasn't the immorality of slavery and concern over human suffering that led to the civil war. It was disparity in economic growth and the constraints on growth of wealth among the southern elite. That said, the south was insane to think it could defeat the north in a war. There was no real reason why a smart wealthy southerner couldn't divert wealth into industrial activity instead of land and slaves and so this idea of stifling southern economy is a stilted one. So maybe it was the inability to accept facts that was the real cause of the civil war.
The Mexican War and the territory the U.S. acquired as a result is what pushed the states to fisticuffs by 1861. The Mexican War precipitated the fall of the Whigs and the formation of the Republican Party--the territory acquired because of that conflict and what to do with that territory fundamentally altered American politics at that time.
Many in the South wanted that territory to be formed into various states, and south of a certain line, be eventually admitted as slave states, thereby increasing southern political voting blocs in Congress. Many former Whigs, new Republicans, Free-Soilers, and Know-Nothings believed in previous laws banning slavery from any future western territory gained by the U.S. Enter Stephen Douglass and the concept of "popular sovereignty" was an attempted compromise--that led to bleeding Kansas and catapulted John Brown and his ilk to terrorist status. He then led his failed plan to emancipate by force Virginia's slaves, was captured and then executed. All across the North, people in sympathy for Brown rang out church bells, and almost the entire South was aghast that northerners would celebrate such a lawless cutthroat.
By the time that Lincoln emerged as frontrunner in the upstart Republican Party's ticket in 1860--a candidate that did not appear on the ballot in the southern states--the political system that existed in an earlier period all but ended. Four viable political parties vied for the presidency, which enabled the upstart GOP to win. At that point the die was cast, and shortly after the election, South Carolina met and voted to leave the American Union.
Why were Republicans anti-slavery? Why was Lincoln so hated in the South? These points are difficult to explain so long after the fact, but Lincoln abhorred slavery. Growing up in Kentucky and Illinois, right on the border between slave and free United States, he saw first hand how slaveholders, usually absentee landholders, held an undue competitive advantage over folks either too poor to own slaves, or morally opposed to it. A team of slaves could move into newly purchased land, clear it, and put it into agricultural production faster than a family of homesteaders could do. And that angered anti-slavery people like Lincoln.
But the Republicans and Lincoln were merely anti-slavery, not abolitionists. The GOP built its party on resisting slavery and its undue competitive advantages in the newly acquired western territory gained by virtue of victory in the Mexican War. They didn't want to end slavery where it already existed. Rather, put it on the path toward reduced influence in American politics and to equal out the playing field in the West. The South was having none of this and recognized that its political influence in national politics was hinged to enabling newly formed states to enter the American Union as slave states.