So, last night's meeting on fun was a good one. But we didn't get any answers, really. So here's my suggestion as to why we find some things funny.
The nearest parallel I can find to humour is being ticklish. We laugh when someone tickles us. Indeed, if a jokes makes us laugh, we might say, 'that tickled'.
So, we find something fun when it tickles the mind. But everyone's mind has different ticklish spots. There may well be spots where everyone is ticklish-- perhaps the mental equivalent of the armpit
-- but apart from that where people are ticklish differs.So there are some things we will all, always find funny. How funny of course is different.
On top of that we learn to be ticklish in certain parts of the mind from the environment. So whilst I naturally find incongruity and irony funny, being a Brit means I also have a ticklish spot for toilet humour.