This is a honest question, why did he stop playing?
For the same reason you have that avatar. Insecurity.
Fischer wasnt insecue
Yes, he was. He spent his whole life working on a singular achievement, and once he had it, he would not risk it or let it go, and lost it anyway. That is insecurity of the highest order. We're talking Gollum's "my precious" levels of insecurity...
this is a more sad quote from Fischer :
"I don't keep any close friends. I don't keep any secrets. I don't need friends. I just tell everybody everything, that's all."