People equate singing and dancing with being happy. I don't disagree, but the problem comes when I have a song in my head, and I have to dance with it, but I'm not particularly happy. So I'll be dancing around and someone will inevitably say, "You're in a good mood." Nope, I'm not. I just have a song in my head that must be danced to.
Current song in my head:
"The Rockafeller Skank" by Fatboy Slim