What makes them good is the same thing that made us - nature. And in the human case, the mental pain of one's self or of others is bad, since we live in the world of information about our own and others' feelings. Pain is bad. That's why it's what you feel when you need to move your hand off the hot stove. Pleasure is good, which is why it's what you feel when you've done something nice.

Is our children learning? (That's supposed to be funny.)