I don't think I'm what the average person would call depressed, but I definitely have my low points. When I'm cycling regularly, generally commuting, I feel a lot better. I guess it's partly the exercise, the endorphin rush you get from it, but I definitely feel happier overall when I'm cycling. Of course going out on group rides is also great, since it gets you out and about other people as well, which is always a good things (well for me anyway!)