Most people think people act based on how they feel. But sometimes it is our actions that govern our emotions. What you show and do physically eventually can override your emotional state, regardless how bad it is.
So.. if you're happy, celebrating, invigorated, show a smile! If you're mad, sad, feeling rejected or frustrated, still smile anyway! It may not solve your problems instantly, but that expression of good feeling on the outside, albeit a bit forced at first, will soon manifest within and make you feel better.
There are exceptions of course, but in general, if you feel negative on the inside, bringing out that same negativity can just make things worse. Give it a try. You'll be surprised how acting differently and positively can always turn things around!