Transcribed from a bit where he talks about his characters, who are not blissfully happy rodents:
"The characters in my books don't realize they're funny. They're taking their lives very seriously. The pigeon who wants to drive the bus, he doesn't think that this is a funny book. He thinks it's tragedy."
And don't click out when they say bye, because there's a little after-credits treat that is so distinctively Mo Willems.
Found via Mo Willems's (Willems'? Willemses?) blog.