Some people will never learn to be grateful or reciprocal but that should not make us to stop doing good and hopping for the best, although protect ourselves from abusers is also important.
To me a key issue is that despite our values are stable we should see "being a helper" or " being a teacher" as roles more than fixed features. Thus, sometimes we can be "the one that needs help ", or "the one that cannot say 'thanks', or even "the one that is behaving like a jerk and need to be forgiven"