Been reading a bit around the topic of reciprocity recently. This seems to be a pretty deep-rooted facet of human behaviour, in both its positive and negative forms. That's why marketing drones seek to use it against us. It's also a very good strategy to adopt in Prisoner's Dilemma (provided it's an ongoing relationship), as famously put forward by Robert Axelrod.
It does suggest, doesn't it, that the desire to reciprocate, including the indebtedness we feel towards people who have done us a favour, is there through a process of natural selection. Survival of the fittest - as in most suited to the environment - actually favours co-operation for mutual benefit. The evolutionary process presumably has led to the feelings we experience that lead to reciprocal behaviour.
Two other things worth noting. Firstly, and most obviously, reciprocation doesn't just mean returning the favour, it also means responding to provocation. Hence the finding that people are willing to incur costs in order to punish someone who defects. This is an eye for an eye, not turn the other cheek.
Secondly, if we apply this approach to life in general then arguably selfish people - those who always defect - will never have their approach disproved. If they always defect they will never experience the worst possible outcome (I co-operate, you defect). They may end up in a lot of situations where both parties lose out, but they won't end up feeling like a sucker. The opposite is true for co-operative players. Sometimes they will get turned over by an unscrupulous partner. Making a bit of a leap, co-operative types are going to experience worse individual disappointments, even if they may do better in the long run.