<p>Previous studies have identified the protective roles of hope (as a cognitive reaffiliation resource) and prosocial behavior (as a behavioral reaffiliation strategy) against adolescent loneliness. However, the bidirectional relations between these constructs and their underlying mechanisms remain poorly understood, particularly regarding whether these associations reflect dynamic within-person processes. This longitudinal study recruited a sample of 1214 Chinese adolescents (<i>M</i>_age = 15.46, <i>SD</i> = 0.71; 39.30% girls) and assessed them at three time points over one year. The random intercept cross-lagged panel model showed bidirectional relationships between hope and loneliness, as well as between prosocial behavior and loneliness. Prosocial behavior positively predicted subsequent hope, while the reverse effect was not significant. Additionally, all mediation effects were nonsignificant. Conversely, the supplementary traditional cross-lagged panel model revealed bidirectional relationships among hope, prosocial behavior, and loneliness, along with four mediating pathways and four self-reinforcing loops. These findings demonstrate that the bidirectional relationships between cognitive/behavioral reaffiliation processes and loneliness operate at the within-person level, whereas more complex mediation pathways primarily reflect stable between-person associations. This distinction advances theoretical precision in understanding adolescent loneliness and contributes to prioritizing targets for personalized support.</p>