Like Paul said, no two people react in the same way to the same situation. I heard the same thing from an Afghanistan vet who said he had no major problems with PTSD but many of his comrades did after being in the same situations. He said," He didn't hold it against them, everyone is different."
Patton's slapping incident was over the top and he was nearly sacked over it. I have read accounts of men who were in the thick of it in WW1 and WW2 for months even years and did everything and more that was expected of them. And one day they simply broke and suffered a total mental collapse. A WW2 vet told me a similar story about his sergeant-major who suffered a breakdown in Holland. He said, "He was the bravest man I ever saw, he just couldn't take any more."