I didn't want to get involved in this, but some of you forced my hand.
All the "taxation" and "state's rights" is crap. Historically documented crap.
If you want to know their motivations AT THE TIME, you don't go to some Lost Causer or modern apologist. You go to the original documents - the Articles of Secession.
And they universally said the cause was slavery. All of them. Sometimes phrased as "our peculiar institution." With a few red herrings thrown in.
Read it. Understand.
ETA: This does not mean that the North fought to free the slaves initially. That came later. It was originally about preserving the Union. But Slavery was why the South seceded.
As an example..