You return the favour to someone who has assisted you, and your generosity is greater the more unrelated to you your helper is - at least that is what you do if you are as intuitively rational as a Norway rat. If you are human you might also consider being the first to act, to reward proleptically, perhaps on the basis of an imagined future favour, and you might make the inverse relationship between degree of relatedness and offered reward or self-sacrifice, even more salient, even less calculated, than the rat - you find yourself, in short, loving your enemy. A naturally politic but somewhat counter-intuitive impulse is taken to be evidence of beauty of soul, of a spirituality transcending mere rattish egoism. But such apparent egoism is an invention for the sake of authentic egoism. The character you are is a dispostional construction out of such imaginary scenarios - you get to feel nice things towards him, you learn the usefulness of applied dreaming and other kinds of literature.