Rabbi Shammai teaches: "Receive every person with a cheerful expression" (The Ethics of the Fathers 1:15). When I taught this teaching to a friendly but somewhat moody man I knew, he said, "That's ridiculous. If I am in a bad mood, am I supposed to act as though I'm in a good mood?" I answered yes (unless one is in an unhappy mood because of something truly bad that has happened), because the lack of a friendly facial expression or manner might well lead the person whom we are meeting to think that we are upset with her. Each of us has had the experience of being with someone who seemed upset, and leaving the encounter wondering if we had somehow offended that person.

Joseph Telushkin in A Code of Jewish Ethics: Volume 1