I asked the kids what they thought was better: to make a favor for someone or to receive a favor. They asked for an example. Taking turns with a toy. They said both people could use it. It took me a while to push to the point where only one person could use the toy that day.
I suggested letting the other person have the first turn was a better route. For a number of reasons: making others hapy often gives us happiness. It is good to be generous. And, finally, assuming the other person was "fair", the next time around it would be our turn. So letting others go first is like saving, while going first is like getting in debt.