Co-rumination, defined as repetitive and emotionally focused problem talk between peers, has been consistently associated with heightened risk for internalising symptoms during adolescence, particularly among girls. Despite growing evidence of its emotional and physiological costs, co-rumination remains common within close friendships, raising the question of why this interaction pattern persists. In this theoretical, integrative mini review, we described developmental, physiological, and interpersonal research to clarify mechanisms that sustain excessive co-rumination in adolescent girls. We argue that co rumination becomes self-reinforcing through the interaction of heightened emotional distress, consciously experienced relational reassurance, and unresolved uncertainty. While early stress appraisal processes during corumination may operate largely outside awareness, the reassurance, validation, and relational closeness experienced during and after these interactions are accessible to awareness and shape learning and future expectations. Building on this framework, we outline the working mechanisms of the recently developed Happy Friends, Positive Minds intervention program that focus on interrupting the relief-without-resolution loop and the prevention of co-rumination and associated anxiety and depression symptoms, while preserving disclosure and relational closeness within friendships.