Rabbi Yechiel Mordeichai Gordon used to say that a sour face is comparable to an open pit in the public domain.  One’s negative demeanor can be contagious, causing others to be infected with this germ of depression.  The Talmud states figuratively: One who shows his friend the white of his teeth [by smiling to him] does more for him than if he had offered him a drink of milk (Kesubos 111b).  A smile is a signal to a person that he is cared for.  Often, the emotional need to feel wanted is more powerful than physical hunger or thirst. ~ The Pirkei Avos Treasury