The study contributes to the literature on extraversion and momentary happiness by examining processes that might underlie this robust effect. The affective-reactivity hypothesis suggests that extraverts react more positively to rewarding situations as compared to introverts. According to the person-by-situation model, extraverts should enjoy social interactions more than introverts do. Global reports of extraversion were combined with an ecologically valid Day Reconstruction Method (DRM) to assess time spent and happiness of 1364 participants during 13,973 activities. Multilevel results confirm that extraverts (versus introverts) experience a higher boost in momentary happiness when spending time on rewarding – but not pleasurable – activities, especially when rewarding activities are executed with others. These processes partly explain why extraverts are happier in the moment.