Just why somebody's great-grandfather or great-great-grandfather went to war doesn't say much about why war broke out.
Slaveowners saw slavery as threatened by Republicans' electoral victory, so they tried to take their states outside the union.
The way they did that -- without approval from the Congress and with the formation of a new country and violence -- meant war, unless the federal government was exceptionally spineless.
I guess if nobody went, you wouldn't have had a war, so individual motivations are contributory factors, but there was a war before most people's ancestors had any say or made any decisions.