Why aren't people grateful for what others do for them?

I volunteer a lot, and I try to be very generous with my time and efforts in the groups that I'm involved with. Mostly, I just want people to express thanks and gratitude for what I've done for them. Mostly though, they don't thank me -- or their thanks just seem perfunctory. Why is that? Am I wrong to want a little gratitude? Right now, I feel taken advantage of, and I want to tell everyone to go to hell. Is that wrong?