Shammai said: Receive all people with a cheerful countenance. What does this mean? It means that if a person presents the most precious gifts in the world to another person, but with a sullen and downcast countenance, Scripture regards that person as though she presented nothing at all to the other person. On the other hand, a person who receives everyone with a cheerful countenance, even if she gives them nothing — Scripture accounts it to her as though she had presented them with the most precious gifts in the world.

Michael Strassfeld in A Book of Life