I've been having this thought for a while. And it's a line of reasoning that works even if you believe that humans are ultimately self-serving creatures. Heck, it's a line of reasoning that works especially if you believe that humans are ultimately self-serving.
Let's just say that each person always has a choice between two kinds of actions, one which is "good", that is to say, extends a benefit to a fellow human being, and one which is "evil", that is to say, harms a fellow human being.
Under what circumstance is it self-serving to do the "evil" action of hurting someone else?
And if it is not self-serving, is it a rational decision?
Let's just say Bob is a greedy-ass fuck. Aight? He'll do anything he can to get what he wants. Does it benefit him more to extend goodwill in his efforts to collect stuff, or does it benefit him to alienate others in his quest?
Now, common sense says that it would probably do Bob a world of good if he could also extend stuff to others in his quest to acquire more stuff.
To be self-serving or self-preserving is, from what I've seen so far of people, pretty normal! Even among crazy people on the outskirts of society! So wouldn't it make more sense to do good than not?