There are two major points of view with regard to this question.
On the one hand, there is the point of view that says that the South seceded out of a desire to continue to have their "peculiar institution." To people who adhere to this point of view, the South seceded in order to keep slavery.
On the other hand, there are those who say that the South seceded on principle, not because it liked having slavery. To these people, the South seceded because it had a principled love for states' rights. It believed that the Constitution was a compact between the various states (not between all the people) and that the states should have many more rights relative to the federal government.