There are two persons who are rare in the world. Which two? First the one who volunteers to help others selflessly (pubbakari). And second, the one who is grateful (katannu) and helps in return (katavedi). A good person (sappurisa) is grateful (katannu) and helps in return (katavedi). This gratitude, this helping in return is praised by fine people. It is entirely the attitude of a good person.
— The Buddha