I suppose whether we like it or not it's subjective.
I like to think that there are things you can state categoricaly are or are not funny, but if I'm honest with myself then I have to admit that standards are subjective.
There are different reasons why things are funny to some but not to others:
you might "get" the joke or not.
you might be personally upset by the joke or not.
you might be bored by or tickled by its simplicity or crapness
you might feel that it it too offensive or wonderfully crass.
It might make you feel physically sick with nausea or uncontrollable laughter.
It might be insensitive or irreverant.
It might be witty or stuffy
It might be clever or pompous
I've tried to list ideas here to show how every genre of humour that I can think of (off the cuff and I've repeated myself really) is either funny or not depending on how you see it - who you are.
I think a lot of it has to do with the confidence of the joker and te receptiveness of their audience.