The way I see it is that although in most relationships people expect reciprocation, they differ in how strictly the people involved are keeping tabs on the difference between how much each is contributing.

In relationships like the ones described in this article, both parties are paying more attention to whether they're getting what they perceive to be a fair reward for what they are providing to the other, and should the balance ever shift too far out of their favor, they will presumably end the relationship.

On the other extreme, in a relationship between two very close childhood friends or between two deeply romantically involved people, one will often do nice things for the other without (I claim) necessarily checking the books to see all the nice things they've done vs. all the nice things the other's done in the past week before deciding whether to do the nice thing. They just do the nice thing, knowing that in the past the other has done the same for them. Only if there comes to be a very long period of non-reciprocation will the other party wake to the one-sidedness of the relationship and begin to reconsider.