Sigmund Freud explained it this way. He said that as we grow up in society, we internalize the social virtues. This internalization leads to the development of the superego. In general, the superego is pleased when we comply with society’s ethics, and unhappy when we don’t. This is why we stop our car at four AM when we see a red light, even if we know that no one is around; and it is why we get a warm feeling when we return a lost wallet to its owner, even if our identity is never revealed. Such acts stimulate the reward centers of our brain—the nucleus accumbens and the caudate nucleus—and make us content.