Humor is fairly complex, but it often involves dramatic subversion of expectations.

Sometimes seeing people hurt themselves may just fall into this funny-because-it's-unexpected category. Other times, I think we can relate the way people injure themselves to some dumbass thing we did to get hurt and we're sort of laughing at a memory of our own folly. I think it's less about the injury itself, than the manner in which it occurred.

Most other cases probably do have uglier underlying explanations.