If you do a person a favor, you would expect that person to like you more. However, the research shows something different. If you do someone a favor, you tend to like that person more as a result. The reason is that we justify our actions to ourselves by assuming that we did the person a favor because we like them.
This phenomenon is dubbed the Ben Franklin effect, who quipped:
He that has once done you a kindness will be more ready to do you another than he whom you yourself have obliged.
It is easy to understand this Franklin effect: one justify prior interaction with a reason and then take that as a sign for the future.