Why is "Am I the asshole" always popular on Reddit

I think one reason is morbid curiosity about how bad other people can be.

Many of the stories there are just incomprehensible that a person can behave like described in the story and then when met with a boundary act as if they're the one who's been momentously wronged.

Kind of makes one sad how little punishment there is in the world vs how much wrong.

I think people like to give that kind of opinion, and they like seeing it doled out on others. Same way people generally seem to enjoy gossip rags and pile on their opinion when celebrities make mistakes.