Explores research in social neuroscience revealing that our need to connect with other people is even more fundamental than our need for food or shelter. Because of this, our brain uses its spare time to learn about the social world--other people and our relation to them. Argues that our need to reach out to and connect with others is a primary driver behind our behavior. We have a unique ability to read other people's minds, to figure out their hopes, fears, and motivations, allowing us to effectively coordinate our lives with one another. And our most private sense of who we are is intimately linked to the important people and groups in our lives.