Parental Love?

Why do we (supposedly) have an innate sense of obligation to our parents?

Is it because of what they have done for us in terms of shaping our lives? Or is it something that is merely imprinted upon our psyche from young: i.e. that we have to love them no matter what? I have come to the point where I don't believe that the latter can possibly be true. I (perhaps foolishly) would like to believe that the world revolves around reciprocation - How can someone feel obliged or love another if the reverse does not hold true? If we feel an obligation to our parents because of what they have done for us, then would one owe a mother anything if she has only brought prolonged pain and strife to everyone around her? Is the act of childbirth sufficient to oblige a child for the rest of his life?

I feel like I have come to a crossroads with regard to this. The next few days will either see me finally shed my ties, or foolishly grit my teeth and hang on in the theme of blind filial piety. I am inclined to follow the former route.