We've all had the unfortunate experience of doing something for someone and never hearing a simple "thank you" in return. It makes us feel sad, perhaps a little hurt or even a little angry, when we have really gone out of our way to help someone. We may think, "How dare they not appreciate what I did for them!" Google the words "un-thankfulness" or "ungrateful people" or "ingratitude" and you'll get hundreds of thousands to millions of results.
Apparently, it's all too common.