Sefer Bamidbar ends with the laws regarding one who kills a person by accident. He must flee to a city of refuge and remain there until the death of the Kohein Gadol. In Parshas Va'eschanan we read how Moshe Rabbeinu designated three cities in Eretz Yisroel as (cities of refuge)"" (Devarim 4, 41). Chazal (Makkos 10a) explain " " (lit. toward the rising sun) homiletically. Some explain that Hashem was telling Moshe, "Cause the sun to shine for the murderers." Others explain that Hashem was commending Moshe on a job well done: "You have caused the sun to shine for the murderers."
Rav Wolbe (Alei Shur vol. I pg. 190) points out that this was the very last mitzvah Moshe performed before he passed away. The greatest prophet felt it imperative to dedicate his last hours on earth to causing the sun to shine for dejected people. The truth is that his entire life was dedicated to this cause. When he was growing up in Pharoh's palace the first story the Torah tells us about him is, " " (Shemos 2, 11). From the beginning until the end, Moshe's life was a continuous saga of caring for the downtrodden, thereby causing the sun to shine for them.
This concept characterizes our regarding . The importance of causing the sun to shine for others can be seen from a statement of Chazal (Kesubos 111a). "Uplifting another person emotionally is more important than providing for their physical needs." (Avos 4, 20). Responding to a greeting is an act of , while initiating a greeting is much more than that. It is an act which literally lights up another's life. Everyone craves and appreciates the interest that another person shows in them. Even a baby responds warmly to a smile: He gurgles and his face lights up in response. Conversely, if he receives a stern look, he immediately starts crying.
A smile doesn't cost anything and it gives so much. Why not confer them more freely? There is no better time to increase than the three weeks between Shivah Asar B'Tammuz and Tisha B'Av. Smile at someone whom you would normally pass by without showing any sign of recognition. You lose nothing yet you accomplish so much!