How to nullify the expectations?

Buddhist teachers say things like > Do not expect anything from anyone, and also: > Do everything with good heart and expect nothing in return and you > will never be disappointed. Such a wonderful lines to get peace and never get disappointed. But as a human, it becomes very tough for us to follow this. Can someone throw light on this please? How we can remove the greedy nature? When we do a lot for someone, we at least expect basic thing in return. It is hard to accept disagreements or carelessness in return from whom where you have done/thought a lot for him in past.