Deborah Tannen writes that girls/women are very concerned with being liked. This means minimising differences, playing down tensions, and avoiding anything that might make another person feel bad. Smiling is a way of 'watering down' any potential disharmony in a social setting, and reinforcing female group cohesion. Boys/men on the other hand, are according to Tannen much more concerned with status. Males are very aware that they live in a status conscious world. There are Alpha males, Beta males, Delta males, Gamma males and those at the bottom of the hierarchy. How can you tell where you are in this hierarchy? It is simple the more people you take cheek, abuse and orders from the lower your status is. Boys have to constantly test each other to see you will obey them and who will not. If a low ranked male won't do what you say them you have slipped in the hierarchy. That is why teen boys are always calling each other names like '->-bleeped-<-', 'gaylord', 'bitch' and indulging in faux aggressive horseplay. They are signalling to each other that any attempt to push them down in the hierarchy will be not be tolerated.
Men are very conscious of either being 'one-up' or 'one-down' when dealing with each other. A male who smiles is saying 'I openly acknowledge your higher social rank. Please don't push me, or swear at me or beat me up. Look see how submissive I am.' No self respecting male wants to appear weak, passive and submissive so he never shows signs of submission i.e. smiling.
P.S. males do use smiling as a sign of sexual interest so be careful who you smile at.