"When one shows his teeth [smiles] to his fellow man, it is better than giving him milk to drink" (Talmud Ketubot 111b).
How does the song go? “When you’re smiling the whole world smiles with you.” And it’s true. On the whole, smiling at another person makes them smile too (unless they are in a really bad mood).
Imagine passing a smile down a street, "infecting" one person and then another. Unlike a virus, smiling is believed to have great health benefits! A wide range of professionals now believe that smiling not only makes you look better, but actually makes you feel better, perhaps even releasing a small dose of helpful endorphins.
The sages, however, were not focused on the effect that smiling had on the one who smiled, but rather on the one who received the smile.
Aside from the fact that both a toothy smile and milk are “white,” one could say that they are both nourishing. Everyone knows the health benefits of milk – how our bodies need milk's calcium and vitamins. A smile, on the other hand, is most beneficial to the soul.
Receiving a smile can change a person’s entire perspective. More than just changing a passing mood, smiles (sincerely, as is implied by the reference of showing one’s teeth) build self esteem, they change how a person views the world and how a person feels that he/she is viewed by the world.
While a cup of milk is a temporary pleasure, a sincere smile can actually change the world!