Those conventions that voted in favor of secession were quite honest about the fact that their intent was to preserve slavery. That is the cause cited by those who actually split the Union, so I don't know why there needs be any more debate. In the words of the seceding states, it was slavery--not economics, not states' rights--that was the most immediate motivation that they could articulate.
It's even more fundamental than that. It's the arrogance of the Northern states, and their complete inability to comprehend that anyone might choose to live differently than them. Look at the history. Heck, we're still living it today!
You'll find recorded in history that the South fired the first shot, but that doesn't mean that the South initiated the conflict. And slavery was the excuse that was used to rally public support.
Take a few steps back, though, and you'll find that the North and South had "irreconcilable differences," and the South decided to secede. It doesn't matter if you love the South or not, if you support slavery or not, or anything else...it was a done deal. The country had split. The southern states seceded legally, and became a separate country, the Confederacy.
Only when the North invaded their sovereign territory did hostilities ensue...and the South fired upon the hostile invaders from another nation.
Kind of a different way to look at it.
Except that Fort Sumter was federal property and therefore the sovereign earth of the United States, which the Confederacy attacked...
Look, it's a tired debate, and I doubt I'm going to convince you that I'm correct, but take a look at the proceedings of those conventions and look at the causes of secession through the words of the seceders themselves. Anyone who is arguing for 'states' rights' conveniently overlooks the fact that the 'right' in question is the 'right' to own other human beings--an inhuman act that could never be one of those unalienable rights endowed by the Creator.
And the four and a half slave states which stayed in the Union? And the three and a half states which seceded only after the attack on Sumter?