In a nut shell
1: the federalism - state question: southern states believed strongly that federalism and the federal gov't should be limited to its appointed constitutional tasks. Southern states had always been afaird that the federal gov't wanted to expand beyond its limits so trust was rare to come by back then. The federal - state balance was NOT as stable as it is now.
2: fear of encourchment - southern states believed northern states were encourching on their automony with their northern rules and ideas despite the Southern custom (this is pretty much our reason for the american revolution btw)
3: slavery - both sides probably couldn't cared less about slavery itself. northern states were just as racist, but southern states felt that because the northern states didn't need them, they would erode southern autonomy and declare slavery illegal. Its possible the vocal minority in the north influenced the opinion of the south.
4: propaganda - past decade since 1840s saw rise in conspirancies of aggression by north making southern states more sensitive to encourchment. see: Kansas Massacre where mid-west and southern settlers in the state of Kansas literally were shotting at each other to determine whether Kansas should be a slave state.
5: politics - inreasing populations in northern states with immigration was making southern states feel they couldn't stop northern states from doing whatever they wanted. Northern states had same fear about south in the Senate - see: when Texas wasn't allowed into the country until a northern state could be created to balance the senate between slave and non-slave (southern and northern)
6: cultural - they had different cultures. Northerners felt southerners were backwards and southerners were friecely protective of their own (not necessarily a bad thing, fyi)
7: economic: northern states were growing and growing, and the southern states were left in the tracks. Northern states were placing increasing pressure on the country which made the southern states feel uneasy. They were afaird of becoming vassals without any serious state automony to do anything.
8: economic: its not unfair to say Southern political elite were adversely affected by national policies that made sense to the north (gold and cattle and banking), but were not optimal for cotton traders.
9: lincoln: lincoln being elected was appriorately the last straw because the election results show southern states everything they feared. The fact that their political weight is so irrelevant that even though not one southern state voted for lincoln, he still won via electroral college.
10: classic divisions - civil war was more do-able back then because people, especially southerners, considered themselves citizens of their states first. This is a hallmark from independence. National identity was only seriously hashed out after reconstruction and 20th century nationalism. (this is the bead rock and the lich pin behind the American civil war)
11: finally, if I'm missing something. It's only because I'm typing all this on the top of my head.
12: rasism