American philosopher, William James, put it very clearly:
"We do not sing because we are happy, we are happy because we sing."
If you want to be happy, find the happiest song you know and sing it. It works. Not always in the first few moments, but if you keep singing, it works.
Laughter itself can also makes you laugh.
"What is the one thing that makes you feel like singing?"
"What gets you in the mood to dance?"
The next time you ask someone to dance, and he/she says, "I don't feel like dancing," you may reply, "That's because you are not dancing!"
C. Steve; 100 ways to motivate yourself