I want to do work, I want to do good to a human being; and it is ninety to one that that human being whom I have helped will prove ungrateful, and go against me; and the result to me is pain. Such things deter mankind from working; and it spoils a good portion of the work and energy of mankind, this fear of pain and misery. Karma-Yoga teaches how to work for work’s sake, unattached, without caring who is helped, and what for. The Karma-Yogi works because it is his nature, because he feels that it is good for him to do so, and he has no object beyond that. His position in this world is that of a giver, and he never cares to receive anything. He knows that he is giving, and does not ask for anything in return and therefore he eludes the grasp of misery.
–from the Teachings of Swami Vivekananda (pp,179-180)