It may be asked why man is supposed to do good and avoid evil.
The first point is due to self-love, and one naturally wills good to himself out of such love.
Is it man’s nature to love others? I think it is, but a much weaker version of this statement is available, namely that it is man’s nature at least not to hate others and through that hate, will evil to them.
Any kind of action is a costly exertion, to be resorted to when the benefits or revenues outweigh the costs. But harming another human being is such an action, yet it benefits the aggressor not at all, as suggested.
No man has any reason to hate another in the state of pure nature. If Crusoe finds Friday, it is insane for him to decide out of nowhere that his good consists in hating and for that reason killing Friday. He pays the costs of such an action (in fact puts himself in grave danger due to Friday’s self-defense) but reaps no recognizable benefits. But such an irrationality entails precisely that Crusoe hates himself, which we have just seen cannot be the case.