It's not just the stress of combat.
No one prepares you for the mind-numbing boredom and constant humiliation of military life, especially for the lowest ranks. This is the greatest contributor to desertion and unauthorized absence (AWOL) in the peacetime military, in my opinion. Even a glorious death in action seems preferable to a teenager, at least until real bullets start flying.
When I got out of the Navy, I had very interesting and challenging work I loved, a little rank and responsibility, good shipmates and superiors, and I loved being at sea on a squared-away ship. And I still hated it. I couldn't wait to get out.
