The positivity resonance theory of coexperienced positive affect (Fredrickson, 2016) identifies the emotion of love as a collective state. This state, termed positive resonance, is defined by the presence of three key features: shared positive affect, caring nonverbal synchrony, and biological synchrony. The current study examined whether a modest behavioral intervention focused on increasing social connectedness could increase study participants' perceptions of day-to-day positivity resonance with corollary impacts on their tendencies for prosociality and self-centeredness. Adults (N = 416, M age = 33.8) were randomized to one of four study conditions: either of two variants of the social connectedness intervention or either of two control groups. Positivity resonance, prosociality, and self-centeredness were measured nightly for 35 consecutive days. Dynamic multilevel factor models of nightly reports showed significant growth in positivity resonance, relative to a passive control group, for the two intervention groups and higher mean levels of prosociality for one of them. In addition, significant dose-response relations were evident (both between persons and within persons), linking positivity resonance to both prosociality and self-centeredness. The within-persons effect for prosociality (but not self-centeredness) was significantly stronger for those randomized to the intervention groups, relative to both passive and active control groups. Taken together, findings suggest that the affective quality of people's day-to-day social encounters may have implications for community flourishing. Discussion centers on theoretical and practical implications as well as directions for future research. (PsycInfo Database Record (c) 2022 APA, all rights reserved).