We can only take from life when we also give: our duty is the debt we pay for what we have already gained. If we wish to continue gaining, we have to continue giving.
For instance, I am not married, so I do not have any duties toward a wife. A husband, though, cannot say, ‘I have no duty to my wife’. All of us have taken a great deal from our parents, so we will always have duties towards them. We cannot say, ‘Why should I do that for my parents?’ We are only free from duty if we have never made a demand. We cannot demand our rights on the one hand and refuse to fulfill our responsibilities, our duties, on the other.
Duty should be done for its own sake, not for any specific reward. Duty is undertaken because it is dharma.
Remember, however, that any requirement which opposes dharma is not your duty. For example, honesty is an admirable quality. But when honesty harms others, it is not dharma.