This article reviews how behaviors and psychological tendencies in close relationships differ between cultures, and proposes a socioecological framework to understand those differences. Our review of the literature finds that paradoxically, people in individualistic cultures are more actively engaged in close relationships (e.g., higher levels of social support, self‐disclosure, intimacy, and love) than those in collectivistic cultures. From an adaptationist perspective, we argue that one reason for these differences is higher levels of relational mobility in individualistic cultures. In societies with high relational mobility, where relationships are relatively more fragile, more active engagement in close relationships helps individuals to impress potential, and retain current, partners. We emphasize the importance of examining socioecologies to better understand close relationships.