@croydon - My local news has a little segment every weekend about people in the community who help out others in some way and they almost always seem to be very happy. It might just be because they are being interviewed, but I think it comes from giving back to their fellow people.
I've found that volunteering is one of the best things I can do for my own sanity. I'm always much happier when I'm donating my time or support to a cause.
I'm not sure if it should still count as a labor of love, because I'm basically doing it to make myself happier and that's the payoff.