The Law of Obligation, also known as pre-giving or reciprocity, states that when others do something for us, we feel a strong need, or urge, to return the favor. Returning the favor rids us of the obligation created by the first good deed. The adage "one good turn deserves another" is a part of social conditioning in every culture. And, even beyond that, the maxim serves as an ethical code that does not necessarily need to be taught, but nevertheless is understood. For example, when someone smiles or gives us a compliment, we feel a great need to return the smile or compliment. Even when these gestures are unsolicited, we feel a sense of urgency to repay the person who has created the mental or psychological debt. In some cases, our need to subconsciously repay this debt is so overwhelming that we end up dramatically exceeding the original favor. The reciprocity trigger created by the car salesman's water is a classic example of this principle. Most of us keep a mental scorecard of these favors.

The drive to alleviate feelings of obligation is so powerful that it can make us bend toward people we don't even know. Accepting gifts or favors without attempting to return them is universally viewed as selfish, greedy, and heartless. It is often strictly due to this internal and external pressure that people conform to the rule of reciprocity. One university professor chose names at random from a telephone directory, and then sent these complete strangers his Christmas cards. Holiday cards addressed to him came pouring back, all from people who did not know him and, for that matter, who had never even heard of him. I had a student raise his hand at a seminar and said, I know him and he is still getting Christmas cards from strangers over 20 years later. Can you believe people have sent out Christmas cards all these years to someone they didn't even know?

Article: http://psych.colorado.edu/~vanboven/teaching/p7536_heurbias/p7536_readings/kruger_dunning.pdf