MORALITY!
‘THE THEORY OF ETHICAL EGOISM’.
This suggests that the goal of each person’s life should be of his or her self-interest. Afterall, it is long-term happiness that is important, so you should be able to behave in such a way that you benefit in the long run. Everybody should apply this theory, as everyone should maximise his or her own happiness. If everyone looks out for himself or herself then everyone is happy.