It makes the viewer very happy, it doesn't necessarily make them amused, it involves pain and anguish and not many people on Earth are heavily sadistic, what's hilarious is how the people keep willingly doing it to themselves, that's the hilarious part of it all. It's not funny, it's (as Pro correctly suggests) stupid.
Whether it's funny or not is a matter of opinion. Now I get where you're coming from, there is shock value and gross out stuff. However, the satire has a lot of wit beyond the surface. In contrast, look at Tim & Eric. All they do are pointless skits with no heart. Jackass, on the other hand, is doing what they love and has a point behind all their material. There's a deep message about loyalty and the courage to do outrageous stuff. I know I probably can't convince you that it's funny but I thought I'd make that point.