Bidirectional relations are likely to exist between adolescents’ friendship involvement and their emotional competencies. Therefore, longitudinal selection and socialization effects were probed in a sample of N = 299 German adolescents in a 30-month study that started in grade 7 (152 boys, M _age = 12.6 years). Cross-lagged panel modeling of the three waves of measurement indicated a pattern of initial selection effects followed by socialization effects, which are best described as spiral effects. Adolescents who were more willing to self-disclose emotions at T1 had more reciprocal friends at T2, which in turn contributed to an increase in their emotional self-disclosure at T3, even after controlling for previous friendship involvement, previous emotional self-disclosure, peer acceptance, gender, and classroom membership. Adolescents with less adaptive coping with sadness and tendencies towards social isolation at T1 were likely to have fewer friends at T2, which in turn intensified these reclusive tendencies at T3. Upward and downward spiral effects are discussed.