Hmm, I wonder why...?
I think Foote put it best when he said in his book that for the most part your average Johnny-reb and Billy-yank fought for many of the same reasons. They fought for glory and adventure, but also because they were afraid not to fight. The propaganda from the North was that the Yankees were gloriously fight'n to save the Union, and the South was traitorously fight'n to dissolve it. The propaganda from the South was generally the same, but with the inference of wickedness of motive reversed.
The irony was the neither side could really explain the nature of the fight, and few have come close since; except as Foote quoted in his book a captured reb remarked to his Yankee captors "I'm fight'n because you're down here."
Personally I think animosity was the main reason, and which because of its own weaknesses as a reason must attach itself to some other cause. In addition to regaining the trade of the Mississippi valley, and Southern markets for Northern goods, I think many Yankees just felt like fight'n Southerners who they perceived as spoiled, reactionary, and worst crime of all arrogant! And, in addition to fight'n for anything else, Southerners wanted to be free of a people they viewed as provincial, greedy, and worst of all, arrogant!
Hmm, I wonder why...?
Lemme try that again...
Rep. Preston Brooks SC responds to Sen. Charles Sumner MA "Crime Against Kansas" speech circa 1856.