As humans, almost everything we do is a cooperative undertaking, and we always have to be alert to the possibility that we will be cheated out of our fair share of the rewards. Because of this, Evolutionary Psychology predicts that humans will be highly attuned to detecting and handling cheating in social interactions.
What is our response to being cheated? We may try to recover what we we feel we are owed. Most cultures have some sort of judicial mechanism to support recovering damages, and to do so is economically rational. But our initial (and primary) response is emotional. We feel an anger which is much the same as if we had been physically harmed. In addition, this anger is in some sense disproportionate and irrational, in that it may lead us into some confrontation with the person we feel has wronged us, where the possible cost to us far exceeds the loss we suffered (we could be killed.) “It's not the money, it's the principle of the thing.”
Robert Frank considers such a disproportionate “irrational” response to be exactly the sort of behavior that is needed to encourage cooperation, especially when there is no rule of law (see Passions Within Reason).