I can't remember the quote exactly but I read Maurice Sendak had said something to the effect that the filmmakers had created a magical movie, and anyone who didn't agree could go f*** themselves. (Really - I do recall that last bit specifically, because I thought it was so great.)

It is supposed to be a bittersweet story, and I don't think it's really a movie for kids, anyway. I think it's a movie for the adults who can remember all too well the feelings that made us want to run away to another world, the complete aloneness of being young and left behind, or of being witness to your parents' troubles without being able to help. It's a world created solely by a little boy, so of course it's off-kilter, and weird, and he's a troubled and lonely little boy, so when he's king he screws up and can't make things right, however much he wants to, and that's sad. But the moments when things are good for him and his monsters, and for him and his mother, are truly and wonderfully joyous.

This is a story that never could have been a light-hearted romp. And it's perfect just the way it is.