It's not even just the banality of life, it's that military life reinforces behaviors that can be detrimental - or at least contrary - to civilian life, and punishes/subdues behaviors that are pretty much expected in civilian life.

I was army infantry and a couple of people I served with have committed suicide. They each almost certainly had PTSD derived from combat situations, but the second thread that connects each of them was failure to transition back into civilian life.

Many of those who serve and then leave the military are capable of making the transition but there's no small number that need significant help in doing so. I've always thought that the idea of an "ETS halfway house" may help some that have significant difficulty transitioning from a world where every decision is made for you, back into being able to make intelligent decisions for yourself. I've met fellow veterans that, even a couple years out, simply don't know what to do, where to do, or how to move forward in life. Then the demons set in, the same demons many that never wear a uniform face, but that now have their grip on a particularly fragile person that doubts they can "hack it" in the real world. That's tough to come back from.