Studies suggest that smiling makes people appear more attractive, kinder, and by some accounts, easier to remember.
All smiles share something in common: an emotional foundation. Depending upon what the emotion is, the brain sends different instructions to the face. The areas in instigating a polite or voluntary smile (the kind exchanged with a bank teller, for example) are not the same ones involved in a more emotional smile (such as the kind that emerges on seeing a loved one or hearing a funny joke).
However, regardless of what prompts a smile, the results are the same. Both you and the recipient are prompted to have good feelings.