Background: The Transitions Catalyst Model suggests increased drinking during young adulthood is due to the notion that alcohol facilitates friendships and romantic/sexual relationships during a developmental period when these relationships are highly valued. However, little research has tested the utility of this model. We examined (1) whether young adults reported greater drinking and related consequences on months when friendships were more important to them or when they were dating casually, and (2) the extent to which social drinking motives explain these associations on a given month. Methods: Data were drawn from 752 young adults (ages 18-23 at screening) living in the Seattle, WA area (56.4% female). For 24 consecutive months, surveys assessed past month alcohol use and consequences, social drinking motives, friendship importance, and dating/relationship status. Bayesian multilevel models were conducted, adjusting for time-fixed and time-varying covariates. Results: Analyses included 11,591 monthly observations. Between-persons, greater average friendship importance was associated with greater drinking. On months when participants reported greater friendship importance than their own average, they reported greater drinking and alcohol consequences. Those who reported more months of casual dating reported greater drinking and consequences on average. Relative to casual dating months, participants reported less drinking during months they were single or in a relationship and fewer consequences during months in a relationship. Associations were partially accounted for by social motives. Discussion: Findings support the Transitions Catalyst Model. Effective strategies for reducing drinking and associated risks among young adults include brief interventions focused on how social drinking motives and relationships relate to drinking decisions.