There once was a psychiatrist in New York who worked with patients who were depressed. While he provided the usual treatment, he would add something out of the ordinary. He would ask all his patients to do a mitzvah.
A mitzvah is a good deed, but not just any good deed. A mitzvah is a good deed where you are not expected to get anything back. You don’t expect an acknowledgment: it doesn’t give you points; you don’t get a reward; and you might not even get a thank you. You just do it because it’s the right thing to do.