Previous research has found that people in individualistic cultures provide more social support than those in collectivistic cultures. Based on a socioecological approach, we propose that this cultural difference is due to the availability of abundant opportunities to replace interpersonal relationships (i.e., high relational mobility) in individualistic cultures. In order to retain one’s valuable relationships in such societies where relationships are fragile, people need to actively attract their partners; social support is one of such relationship-retention strategies. In this study, Canadian and Japanese participants read hypothetical scenarios where their closest friend either provided or did not provide support to them, and reported their evaluations of the friend and their intention to distance themselves from the friend. As predicted, Canadians perceived greater relational mobility in their immediate social environment than did Japanese. In addition, both Canadians and Japanese evaluated their closest friend negatively when they imagined that the friend did not provide social support. However, in the same situation, Canadians reported greater intention to distance themselves from the friend than Japanese. This supports our hypothesis that support provision is a necessary behavioral strategy in high relationally mobile social contexts where interpersonal investment is critical for relationship retention.