I can't seem to laugh at things people laugh at?

I mean I don't find mainstream things funny. Like stand ups? I have never laughed once and I have seen a full episode of Tosh.0 and a full episode of Dave Chapel. I have also watched South Park and Family Guy. Still no laughs! THE ONLY SHOWS OR MOVIES THAT MAKE ME LAUGH are the spoofs like the Scary Movie ideals; Date Movie, Meet The Spartans, Comebacks, Airplane, etc.. I actually die laughing at those. But nothing else on TV or any other movie. Not even American Pie and stuff like that... Why is this?