As a Vietnam veteran suffering from PTSD at a 70% rate as per the VA, I think I can answer this question for you.
I was a squad leader in the 173rd Airborne Brigade, in the infantry. My MOS was 11B4P That is rifelman, infantry, Sergeant, parachute qualified.
I don’t do “war stories” often, but I’ll tell one just to give you an example of the traumatic things we faced.
About 9 of us were chosen to spring an ambush on the VC who were using a nearby village. This village was situated in a sea of rice paddies about 800 to 1000 acres in size with it being in a small oasis in the center. The first problem came when the chopper sat us down in the only other oasis in the rice paddy about 300 yards from the village and they put us there before dark, which means the VC could see us being brought in. After dark we snuck (as best we could) to the village and set up our ambush next to a trail behind a small hedge row. Of course the VC knew where we were, but instead of avoiding the trail, they started lobbing hand grenades at us. It was a cloudy night with no moon or stars, so it was pitch black. There was no muzzle flash, so we couldn’t tell where the grenades were comming from. We would hear a “thud” when it hit the ground and a second later it would explode. A grenade has a 5 meter killing radius and will seriously injure you at 15 meters away from the blast. They threw 8 or ten grenades at us that night. 7 of the 9 of us were killed or seriously wounded that night. Myself and our medic were the only two who survived intact.
That was one night in combat. We averaged a firefight about once every 3 days if I remember correctly.
If you multiply that experience by all of the times in contact with the enemy, you can possibly understand why some of us have PTSD.
PS. PTSD doesn't make me want to grab a gun and go on a rampage at everything that happens in life. For me, PTSD mostly won’t let me sleep at night
