"The Torah ideal is to greet each and every person with a pleasant facial expression." (Tomar Devorah, ch.2) When you greet someone in a friendly way, you never know what a positive effect you will have. A certain individual who greeted everyone with a smile and kind words was approached by someone and told, "You saved my life." The person went on to tell how he'd suffered a number of serious setbacks and was contemplating suicide. He felt totally alone and depressed and felt that no one cared about him. Then this fellow greeted him with a sincere smile and a cheerful voice. This immediately lifted up his spirits and he was resolved to continue living.
Experiment just for today. Greet everyone you encounter with a smile. If you need to, visualize yourself greeting others in a cheerful and friendly way. Note how they react. A smile is contagious and can potentially transform the mood of thousands!
(Tomar Devorah, ch.2; Gateway to Happiness, p.26)