As I posted earlier, what's wrong with the idea that most of the combatants went off to war as kind of a serious lark? Most young people North or South never traveled further than ten miles from their birthplaces and the war created an adventurous escape from the monotony of life--even with the costs associated. To fight along "My people" (Foote), or "to see the elephant" was motivation enough--read the letters of farm boys that are all of a sudden not just in the state capitol signing up but training and then read the letters as they travel from state to state--at least in the case of recruits in the West. They describe exciting new things and compare things and places they know with the new ground they find themselves on--comparing farms and crops, houses, and, yes "the girls"
I'm sure the existence of state militias also added to the "why" they fought--many of these men trained for years in these militia groups and the war was exactly what all that training and "preparation" was about.
I cannot imagine that, at the outset, there were long discussions among 17-20 year old young men about the "morality" of slavery and the necessity of preserving the Union. The American male, then or now don't need economic theory or political theory to motivate them to fight--Americans simply fight--it is in their nature then and probably is still in that nature.
I would imagine much of the reason is far simpler, your friends were going, your brother, father, uncle, cousin were going and, by god, you were going--some of it was no doubt "duty" but I suspect most went out of curiosity and simply to get out of Dodge and get paid to boot!