Why is watching someone take one in the nuts so funny?

Watching a program on TV about 20 most stupid something. Without fail, there are several people who get their genitals punished severely. Why do we find that so funny? (Save for at least they are not your own nuts pretending to be tonsils) It seems to be independent of nationality. Sometimes these fools purposely abuse their genitals. One guy threw like a heavy bag off the roof directly on another guys crotch. I’m a little ashamed to laugh, although I do cross my legs.