Although fear of missing out (FOMO) has become a widely experienced phenomenon, the specific social situations and cognitions driving the FOMO experience have not yet been closely studied. Across seven experiments (N = 5,441), we find that FOMO occurs when people miss events involving valued social groups and is driven by the perception of missed bonding and concerns about how this may negatively affect their future relationships. People feel greater FOMO when they miss events involving valued others (vs. strangers or irrelevant social groups) and when events foster social bonding (vs. individual activity)-even when the events themselves may be unenjoyable. FOMO is further intensified when concerns about one's future social-group belonging are elevated, either stemming from situational triggers (e.g., social media photos) or one's chronic anxious attachment to their social group. Notably, these concerns are exaggerated when considering the social costs of missing an event for oneself (vs. a friend). Given the social underpinnings of FOMO, reaffirming one's social belonging by reflecting on past social connection provides temporary relief. By revealing a novel, situational antecedent of FOMO and the underlying cognitions, this research demonstrates that current well-being is informed not only by current and past feelings of belonging but also by hypothetical projections about one's future social belonging. Taken together, our findings highlight the importance of social bonding in experiential consumption, identify new determinants of FOMO, and lay the groundwork for simple interventions to mitigate FOMO and its maladaptive consequences. (PsycInfo Database Record (c) 2024 APA, all rights reserved).