Honestly, I'm still searching. I tried Steve's exercice, but it didn't really work for me.
As for now, even though I'm still thinking about it, I have come up with something along the lines of:
"Making people feel good"
If you make people feel good, they get cheerful, and that vibe flows around, and more people feel good... Like a chain reaction or something. Also, I tend to find that people are more receptive, more open, and accomplish greater things when they feel good. Plus, you made someone feel good, so you feel good about that...
However, that is a great challenge for me, as I'm an introverted and have a tendancy to be shy... So I have to work on that in order to get around my "purpose" of the moment!