• Smiling Makes Us Attractive
• Smiling Changes Our Mood
• Smiling Is Contagious
• Smiling Relieves Stress
• Smiling Boosts Your Immune System
• Smiling Lowers Your Blood Pressure
• Smiling Releases Endorphins, Natural Pain Killers and Serotonin
• Smiling Lifts the Face and Makes You Look Younger
• Smiling Makes You Seem Successful
• Smiling Helps You Stay Positive

SOURCE: http://longevity.about.com/