Abstract: Previous research has shown that people in individualistic cultures feel higher intimacy toward their friends and romantic partners than those in collectivistic cultures. Why do such cultural differences exist? Based on a socioecological approach, we propose that intimacy is a psychological process underlying relationship-maintenance behaviors (e.g., social support) and is more adaptive in societies where abundant opportunities are available to form new relationships (i.e., high relational mobility). In such societies, there is a high possibility of one’s partner leaving for alternatives, and thus, people need to actively maintain their partner’s attraction to themselves. In this study, Canadian participants completed measures of relational mobility, intimacy, and social support. As predicted, the indirect effects of relational mobility on social support through intimacy were significant. Thus, Canadians who perceived higher relational mobility felt stronger intimacy towards their romantic partner, which then predicted greater social support provision to the partner.