The present research evaluated the social, affective, 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 changes over time in tandem with perceived social integration (PSI), and whether PSI is associated with future activity indirectly through sociality during physical activity, positive affect during physical activity, and positive spontaneous thoughts about physical activity. Adult participants (N = 226) reported daily on their PSI, physical activity behavior, and affect during physical activity for 11 weeks. Once every 2 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' PSI increased, so did their positive affect during physical activity. Further, structural equation modeling of specific reports revealed a significant indirect effect of PSI on future activity through sociality during an intervening instance of physical 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 affective and cognitive mechanisms by which social processes may contribute to positive health behavior change and maintenance: namely, positive affect during physical activity and spontaneous thoughts about physical activity. Beyond its utility for evaluating theory, the present study may inform subsequent research aimed at developing sustainable behavior-change interventions. (PsycInfo Database Record (c) 2020 APA, all rights reserved).