"I don't sing because
I'm happy because I sing."
Doesn't he look happy?!
Disclaimer: I don't think this applies to medical conditions such as: clinical depression, bipolar disorder etc. It's a little too simple for that, but I have found that in times of trouble acting happy can lead to feeling better!
I'm taking William James word on this today though. It is January after all and I believe happiness may reside in Hawaii. (Although I wouldn't know I've never been--tragic I know.)