More than 2,300 years ago, Aristotle wrote about the importance of friendships to society, and other Greek philosophers wrote about emotions and their effects on relationships. Although research on relationships has increased dramatically over the past few decades, the fact that these revered ancient philosophers included them in their writings illustrates the important place interpersonal relationships have in human life.Daniel Perlman and Steve Duck, “The Seven Seas of the Study of Personal Relationships: From ‘The Thousand Islands’ to Interconnected Waterways,” in The Cambridge Handbook of Personal Relationships, eds. Anita L. Vangelisti and Daniel Perlman (Cambridge: Cambridge University Press, 2006), 13. But how do we come to form relationships with friends, family, romantic partners, and coworkers? Why are some of these relationships more exciting, stressful, enduring, or short-lived than others? Are we guided by fate, astrology, luck, personality, or other forces to the people we like and love? We’ll begin to answer those questions in this chapter.