The self-expansion model posits that individuals are fundamentally motivated to expand their sense of self. It is proposed that approach—but not avoidance—motivation underlies self-expansion and that approach-motivated individuals should be especially interested in self-expanding with a person who provides many novel resources, identities, and perspectives. In Studies 1, 2a, and 2b, correlational evidence that self-expansion is associated with both relationship-specific and global measures of approach motivation, but is unrelated to avoidance motivation, was found. In Study 3, experimental evidence that approach motivation increases sensitivity to self-expansion opportunities, such that individuals high in approach motivation are especially attracted to targets who offer many expansion opportunities and unattracted to targets who offer few expansion opportunities was found. Taken together, these studies provide evidence that self-expansion is rooted in approach motivation.