This is a very serious question to ask. The answers needs to come from people who are genuinely concerned in finding out why. One thing that is recongized though. Many a soldier from many a war has committed suicide on their return to their peacetime existence. Here, in Australia, the suicide rate from returning Vietnam War veterans was high, I'm led to believe.
Post traumatic stress is something which can build up gradually over time. A veteran returns home. He appears to be at a loss about how to live now. He might choose to wander the world. Or escape from it with alcohol or drugs. He might work his way through it and come out a wiser, better adjusted human being. Then again, he might not.
Asking a man (or woman) to go away and to kill people (and that's what the bottom line is) who they don't know and who have never done anything personal to them, is asking a man to become a 'legal murderer.' This is sanctioned by government decree. But in the deep subconscious of the man who has to do the actual killing, things don't always sit well.
Still, as long as governments can get young, immature people to go out and do this sort of thing for them, whilst they stay at home and play loyal patriarch, it will continue.