Because They've Earned It

I don't believe in fair. It is a silly concept that only belongs on board-games.
Each person's worst pain is the worst pain in the world as far as they are concerned.
It drives me nuts when I try to comfort someone when something has happened to them and they say some shit like "I have no right saying anything to you with all that you have been through/go through".

Yelling can release endorphins... it does help in extreme cases. Most yell before they need to or because they are surprised because they did not realize how bad something was going to hurt or (mostly IMO) because they are not used to pain.

My Drs are always amazed at my pain threshold (not a good thing, it is a problem with my meds) due to having always been in pain and my brain/nervous system functions differently. Also, I'm just very stoic about it.

Some pain meds do not work on me, I had a root canal with no working meds once, he did not believe me, did not know my history... that was very bad.