Why do some people think that a film has to be funny to be good?

Because for some reason people have to hear a fart joke every five minutes, to remain entertained. I didn't see gnomeo and juliet, but how to train your dragon was a fantastic movie. Animated movies are aloud to be serious. But for some reason everyone expects comedy when watching them, and then gets let down when itas not funny. How to train your dragon was never even classified as a comedy. Its more of a fantas/adventure movie.