Why do I not find funny the things which others do and find funny things which they think are not?

It is not invariable, sometimes I do find the same things as others to be funny but a lot I will be trying to remain serious when other people have no inclination to laugh or I will be pretending to laugh at something because I do not wish to be rude but others are genuinely laughing. Sometimes I find the most horrible things to be funny and I have to fight to remain serious so that I will not upset others. The other day I was driving by a man on a bicycle who ran into the kerb and fall off, and all of the people in the car act like they are very concerned and I have to stuff my face in my elbow into the corner because I am uncontrollable LAUGHING of the sight of it! Am I evil or bad or something to laugh? I do not wish the man to be injured but just to see this happen was very much hilarious to me.