When my siblings and I were younger and in a bad mood my mom would always tell us to smile, it would make us feel better. Of course we would do the opposite and look at her like she was crazy. Just because we smile doesn't mean we are going to be happy. Then she would usually do something to make us smile and laugh and then say "see."
It turns out that expressions influence your emotions. So my mom wasn't lying when she said it could help our mood. Smiling cannot bring you out of a bad mood but it will definitely help some and get you to try to think positive. I thought this was very interesting when reading about it. Although she wasn't completely right she had the gist of it.