Couldn't tell you. I remember being VERY stressed in Iraq after a few months when we were always so ready to kill bad guys and they never, ever showed their faces. IEDs were somewhat common, but even that was rare. The stress was mostly self-inflicted, to be honest. I was trying to will something to happen, but that wasn't going to work. Once I accepted that, the deployment got much easier.
Some of these guys have experienced some pretty bad stuff, but then they're immediately told to relax and not do anything to anyone (lest someone take out his aggression on the nearby village with numerous unknown enemy). It may be the right thing to say, morally, but that crap is typically the last thing you want to hear after your buddy just got splattered all over the road.