The stress of being forever on high alert, making fast choice after fast choice, takes its toll. Decisions must be made with a cool mind but the action is high intensity, and that needs tunnel vision. Despite what Kevin Costner and Mel Gibson may have you believe, it is impossible to keep up. It is a punishing and brutal exercise which will break you.
Remember the dude who had to chop off his own arm to save himself because he got it stuck between a literal rock and a hard place? In the apocalypse, situations like that are your whole life. Your daily schedule will be something like:
1) Wake up.
2) Kill someone's beloved pet for breakfast.
3) Fight over the carcass with an emaciated kid.
5) Cry with gun in mouth until exhausted.
"Hm. I'm in the mood for gun metal for breakfast."
The assumption is that this will become routine, but the post-apocalyptic world is unpredictable. The brain can adapt to almost anything but only if that thing happens regularly. Almost every day, a survivor would have to make a choice that would have brought them to the point of tears and vomit in their pre-apocalyptic life. Right now, in our modern world of ultimate comfort and convenience, do you have a hard time deciding what to watch on Netflix? Then good luck making a decision when you're starving and one of your fellow survivors starts to look like a giant roasted turkey.