Today it was a dentist who didn't charge me for the work he did, saying he was happy that I would no longer be in pain.
Tomorrow it could be me, or it could be you. A smile costs nothing and uses less muscles than a frown does and it could make the day of someone who is feeling utterly awful.
Smile, even if you feel awful; it will make you feel better even if you think it won't. Laugh; it really is the best medicine. Be happy, because if you're happy, then the world seems like less of a dark place.