Can you think of a time when someone did something nice to you for no apparent reason? It may seem like this happens rarely.
And maybe that is why when it does, it really stands out. When someone does something to help us in some way, we feel grateful.
And when we feel grateful, we also feel like we want to do something for the other person. What you are feeling in these situations is the law of reciprocity.
Read more on lifehack.org