No matter how much you practice being positive, and looking for the bright side, there are certainly times it's appropriate to feel sad, or blue. The obvious times are a death, the loss of a job, or an upset with a friend or loved one. But what do you do if you find yourself in a funk and there's been no major event to blame? If the normal things you would usually do don't work, I say embrace feeling down. Maybe it's the moon, your monthly cycle (if you're a woman of course), or just simply stress.
I'm usually so happy and up, that when I'm blue I actually feel guilty. Even with a clear and legitimate reason to be sad or angry, my logical side fights with my emotional side. When my emotions win, I give in. I allow myself a day or so to feel sad and it typically falls away on it's own. If all else fails, remember that we can't fully appreciate the good days without an occasional bad day to gauge it by.