Help is a matter of mind as well as heart.
Obligation can be a subtle form of abuse by which the helper showers unasked for favors upon his victim. The 'helper' feels better about himself while the recipient feels worse. This is no help at all.
Do not expect gratitude. Sometimes it will come, but do not expect it. If we remember that we give because we enjoy giving we will not become resentful of the people we give to.