You wrote this 5 hours ago, so it wasn't me, but I had a great time doing that exact thing just a bit ago. The reward you get is whenever someone hearts you back, which happens frequently.

Maybe we really do live in a cooperative world.

Sure we do. This is pop sociology, but it makes an interesting point:

http://www.forbes.com/2006/09/22/trust-economy-markets-tech_...

Greetings from another heart person! I also met a guy who drew me a kitty at the end, which I thought was nice.