Nobody can quite state it like Oswald Chambers -- and this excerpt from his devtional for today hit home with me:
"If our devotion is to the cause of humanity, we will be quickly defeated and broken-hearted, since we will often be confronted with a great deal of ingratitude from other people. But if we are motivated by our love for God, no amount of ingratitude will be able to hinder us from serving one another."