An interesting theory that facial expressions affect blood-flow to the brain and are not just results of emotions. The assertion is that these blood-flows affect our emotions. So SMILE! :-)
Zajonc, R. B., Emotion and Facial Efference: A Theory Reclaimed, Scince, 1985, 288, 15-2
He also asserted that elation follows the smile, not the opposite. The blood flow changes caused by contracting the facial muscles in the smile alter cerebral blood flow and cause an emotional change. He extends this reasoning to account for all kinds of other bizarre facial habits associated with emotions -- wrinkled forheads, rubbing one's eyes, hand on forehead, pulling earlobes, licking lips, etc.