According to Mark Stibich, Ph.D., on Longevity.About.com the Top 10 Reasons to Smile are:
 
1. Smiling makes us attractive
2. Smiling changes our mood
3. Smiling is contagious
4. Smiling relieves stress
5. Smiling boosts your immune system
6. Smiling lowers your blood pressure
7. Smiling releases endorphins, natural pain killers, and serotonin
8. Smiling lifts the face and makes you look younger
9. [...]