It is a gesture instinctively evolved by some of the apes as language of appeasement. Baring the teeth while frowning has the opposite meaning and is a sign of aggression.
I recently read a nature book where a whole chapter was devoted to natural sign language by humans. It described how in the 60s new tribes were still being discovered in the Indonesian region, and people who had never seen 'white man' before were able to communicate with signs, smiling being not only a reaction to something funny but also agreement. The tribes that were part hunters also had an extensive semaphore language where they could clearly communicate with each other for over 200metres+.
I think a smile is an inborn relex action to something which makes you happy or agreeable. Perhaps visual images, as well as pleasant thought, stimulate endorphins - which trigger the nerves in the facial muscles to express these feelings as a smile?
wildwood....you wouldn't happen to know what book that was do you? i'd like to try to find it. thank you so much. thanks for the answers guys. and if any of y'all have anymore answers, i'd love to hear them, or any suggestions as to where i could take my research, because, i'm quite frankly not having much luck. lol.