Linking to the FAQ or previous answers is not intended to discourage further questions or discussion. It's just a timesaver.

In this case, there's the FAQ section "The Civil War and Slavery" at https://www.reddit.com/r/AskHistorians/wiki/civilwar#wiki_the_civil_war_and_slavery . /u/Borimi had a shorter answer, but the first two articles by /u/Georgy_K_Zhukov are pretty definitive and well-sourced (20-item bibliography, many quotations from primary sources). The primary answer under the second item, "Why was there a Civil War? Why could that one not have been worked out?" (at https://www.reddit.com/r/AskHistorians/comments/68myvr/why_was_there_a_civil_war_why_could_that_one_not/dgztopa/ ) is newer, and there are many more replies on various other topics there. Though at most he must have just polished the previous version in the first item, "Was the American Civil War about more than just slavery?" (at https://www.reddit.com/r/AskHistorians/comments/3edss0/was_the_american_civil_war_about_more_than_just/cte2mj9 ).

You could look at the declarations of secession for the individual Confederate states. They make it pretty clear:

Georgia: "For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery."

Mississippi: "Our position is thoroughly identified with the institution of slavery."

South Carolina: "[The non-slaveholding] States have assumed the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery"

Texas: "[Texas] 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."

Virginia: "The people of Virginia ... having declared that the powers granted under the said Constitution were derived from the people of the United States, and might be resumed whensoever the same should be perverted to their injury and oppression; and the Federal Government, having perverted said powers, not only to the injury of the people of Virginia, but to the oppression of the Southern Slaveholding States."