As a indie dev, the nice comments are the ones that keep me going. Buying my commercial games is of course the big way to support me financially. But to keep me sane, to keep morale up, and for me to stop thinking I'm worthless, all it takes is a dumb little thank you on twitter or a tiny e-mail with a positive thought and a smiley to get me going again. Stuff like that gets me out of bed, gets me working, and overall allows me to believe in myself, that what I'm doing with my life is worth it because it's affecting someone in a positive way.
And then there are the really big thank yous - the fan arts, the lengthy e-mails, the fan videos, the heartfelt deep comments buried on a forum somewhere about how one of my games helped someone cope with the loss of a loved one and move on... That stuff doesn't happen as often, but it does. And it touches me, much as I had touched the player. And it's at those moments that I know for a fact that what I'm doing is worth it.