To everyone who is clueless about the joke - It's a line from the movie Donnie Darko. Frank is the big scary rabbit (see avatar).
At one point in the movie..
Donnie says to Frank "Why do you wear that stupid bunny suit?"
Frank then turns to Donnie and says "Why are you wearing that stupid man suit?"
Donnie Darko FTW, my favorite movie hands down. Here's the scene from the movie: