Six Ways to Make People Like You – Principle 2
Your smile is a messenger of your good will. Your smile brightens the lives of all who see it. To someone who has seen a dozen people frown, scowl or turn their faces away, your smile is like the sun breaking through the clouds. Especially when that someone is under pressure from his bosses, his customers, his teachers or parents or children, a smile can help him realize that all is not hopeless—that there is joy in the world.
Smile.
* Source: How to Win Friends & Influence People by Dale Carnegie