There is currently quite a few posts about this on here, but of course, I am not discouraging any new answers coming in. I think you should check out the FAQ part about causes of the Civil War to get a better understanding of it. Let me quote /u/AncientHistory here as well, who put it succintly yesterday:

There absolutely were many Southerners that opposed slavery. There absolutely were many Southerners that opposed the Confederacy. Every state in the Confederacy sent troops to join the Union Army. Yet the fact remains, that the presiding cause of secession itself was slavery - and that every soldier that fought for the Confederacy, whether they owned slaves or not, whether they were from the Confederate states or not, whether they believed in the institution of slavery or not, fought for the enslavement of human beings. The Lost Cause was the cause for the continuance and furthering of slavery; the independence of the Confederate states was independence to continue slavery; the freedoms that they felt were denied to them were the freedoms to continue to keep others as slaves.

The reasons behind secession are quite clear if you go to primary sources -- in this case, the official declarations of secession issued by the states themselves. I'll give you a few examples:

From Georgia's Declaration of Secession:

The people of Georgia having dissolved their political connection with the Government of the United States of America, present to their confederates and the world the causes which have led to the separation. For the last ten years we have had numerous and serious causes of complaint against our non-slaveholding confederate States with reference to the subject of African slavery. They have endeavored to weaken our security, to disturb our domestic peace and tranquility, and persistently refused to comply with their express constitutional obligations to us in reference to that property

From Virginia's Ordinances of Secession:

The Federal Government having perverted said powers not only to the injury of the people of Virginia, but to the oppression of the Southern slave-holding States

From Mississippi's Declaration of Secession:

Our position is thoroughly identified with the institution of slavery - the greatest material interest of the world.

(And lest you think I've cherry-picked a single sentence in order to remove it from its context: that's the second sentence of the entire thing. The document continues discussing slavery at length. Read it for yourself; essentially the whole thing is concerned with slavery.)

From Texas's Declaration of Secession:

WHEREAS, The recent developments in Federal affairs make it evident that the power of the Federal Government is sought to be made a weapon with which to strike down the interests and property of the people of Texas, and her sister slave-holding States ...

She was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery - the servitude of the African to the white race within her limits - a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time. Her institutions and geographical position established the strongest ties between her and other slaveholding States of the confederacy.