GEN1329 Rabbi Shammai teaches: “Receive every person with a cheerful expression” The Ethics of the Fathers 1:15.  … If we are aware that we are in a bad mood and find it hard to greet others cheerfully, we should quickly note to the people whom we meet that we are upset about something, so that they won’t feel that we’re irked at them.   That the lack of a smile and a friendly demeanor does often reflect ill will is reflected in Jacob’s statement to his wives, Rachel and Leah, when he tells them that they must leave their father Laban’s house [this verse].  … So important is it to cultivate a pleasant facial expression that the Talmud teaches that “the man who shows his teeth to his friend in a smile is better than one who gives him milk to drink” Ketubot 111b.   In short, smiles are a powerful form of nourishment and inspiration. The Talmud teaches elsewhere: “If you see a student whose studies are as hard for him as iron, attribute it to his teacher’s failure to show him a cheerful countenance.” Ta’anit 8a  TELVOL 1:122-4