When you sing the body releases endorphins, the brain's feel good chemicals.
I have seen it with so many of my students, how happy they feel when they sing.  
According to a study it was found that after nursing-home residents took part in a singing program for a month, there were significant decreases in both anxiety and depression levels. 

Singing can have some of the same effects as exercise, like the release of endorphins, which give the singer an overall "lifted" feeling and are associated ...