The present research evaluated the social and cognitive processes underlying sustained patterns of health behavior as articulated by the upward spiral theory of lifestyle change. Specifically, we tested whether positive affect experienced during physical activity change over time in tandem with perceived social integration, and whether social integration is associated with repeated activity indirectly through sociality during activity, positive affect during activity, and positive spontaneous thoughts about physical activity. Adult participants (N = 226) reported daily on their sense of social integration, physical activity behavior, and affect during activity for 11 weeks. Once every two weeks, they also reported on features of a specific bout of physical activity, including how social it was, positive affect during the activity, and positivity of spontaneous thoughts about physical activity. Multilevel modeling of daily reports over the 11 weeks revealed that as participants’ perceived social integration increased, so did their positive affect during physical activity. Further, structural equation modeling of biweekly reports revealed a significant indirect effect of social integration on repeated activity through sociality during an intervening instance of activity, positive affect experienced during that activity, and positive spontaneous thoughts about physical activity. The findings reported herein provide evidence consistent with the upward spiral theory of lifestyle change and reveal a mechanism by which social processes may contribute to positive health behavior change and maintenance: positive affect and spontaneous thoughts. Beyond its utility for evaluating social and affective theory, the present study may inform subsequent research aimed at developing sustainable behavior-change interventions.