Slight variant: Humour is a form of teaching, in which interesting errors are pointed out. It doesn't need to involve an outsider, and there's no particular class of error, other than that the participants should find the error important.
If the guy sitting behind you starts moaning and grunting, if it's a mistake (e.g. he's watching porn on his screen and has forgotten he's not alone) then it's funny, whereas if it's not a mistake, and there's something wrong with him, then it isn't.
Humour as teaching may explain why a joke isn't funny twice - you can only learn a thing once. Evolutionarily, it may have started as some kind of warning, that a person was making a dangerous mistake, and then getting generalised.