Social exchanges are usually governed by the norm of reciprocity,
which requires that people help those who have helped them.
If a favor has been extended to us, we will be motivated to return the favor.
Conversely, if others have not been helpful to us, we are not likely to be
helpful to them.
Therefore, if social exchanges are fair, the social structure involved
tends to be solid. The exchange reinforces the relationships and
provides each party in the exchange with some needed good.
But if exchanges are seen as unfair, the social structure is likely to be unstable.
A friendship in which one person constantly helps another, expecting but not
getting gratitude in return, is likely to be short lived.