The South was tolerant of the Buchanan administration. It was the election of Lincoln that convinced them there could be no compatibility.
Lincoln was the Barack Obama of his day. 
Extreme Liberal from Illinois, obsessed with Racial issues, and intending to use every executive order at his disposal to undermine what was then existing law. Willing to misuse and abuse the Federal government to advance his own agenda, and to whom no one would stand up and tell him he was exceeding his authority.
His position was "If you want your slavery, you can keep your slavery." But nobody in the South believed him. They thought he was an opportunistic liar who exploited increasingly popular Liberal ideas from places like Massachusetts.
Southerners realized he couldn't be trusted to respect their rights; that he would undermine them in any way he could, and so they decided to preemptively solve the problem by exercising their voluntary right to leave the Union which was formed by their previous consent.
(Just as the Declaration of Independence said any people have a right to do.)