I also believe that den is correct in the location for some who went to one side or the other. Those border states were different from the established 'interior' or core Southern states. They were still trying to get into the 'state' over 'territory' thought. I think my line from Central Florida had a very different idea of it than the Alabama/Georgia/Virginia lines. They also had different economic backgrounds. Though I had none who were in the Union Army, I do think some went to 'preserve' the Union because that was something they believed worth fighting for. I'd think those with family who fought to bring together the 'Union', it was the only reason they needed. I found myself comparing it to my kids. I have 3. All were raised with the very same rules and expectations. Each one is very different. Using that as an idea for the 'joiners', I can see how it was not a one size fits all deal. Just as slavery is not the reason for the war, it's for sure a part of it. There is just too much evidence that many Southerner's did include the slaves as part of their 'family' or close ties to home one cannot say they were as heartless as Stowe tried to say in her book. That entire period is a lesson in contradiction.
I too find that tracing my own family's steps during that period and trying to understand what they were seeing and doing to be a very interesting and eye opening experience. No matter the reason they went, they still faced cannon. They all shot their guns and they were aimed at Union soldiers. A good number faced the demons of the POW experience and only 2 in that catagory lived to tell of it but never wrote anything down on that exerience. I so wish they had.
What we do know for sure is that a whole lot of men left everything to go fight a war. Some thought they'd be home for the harvest. Some were never to come home. The womens stories, diaries, journals, letters that have been published tell us they and their children suffered just as much as their men but in a different way.