Aggressive behaviors can serve different functions, which might be understood by distinguishing between reactive (RA) and proactive (PA) aggression. Few studies were conducted on adolescents’ family precursors and emotional processes associated with RA or PA. Accordingly, the current study compared RA and PA by evaluating their association with adolescents’ attachment to parents and alexithymia. N = 453 Italian adolescents aged 15–19 years (Mage = 16.48; SD = 0.69; 33.6% males) participated in the study filling in self-report measures. Results showed that PA and RA are significantly associated and that PA was higher among males. Moreover, four mediational models were performed to assess the influence of adolescents’ attachment to mothers vs. fathers on RA or PA, considering the mediating role of alexithymia. Gender was included as a covariate. Mediational models’ results showed a direct and indirect effect, through lower alexithymia, of adolescents’ attachment to mothers and fathers on RA. Differently, only attachment to mothers showed a direct effect on PA, while attachment to fathers only an indirect effect, mediated by lower alexithymia, on PA was shown. Findings support the greater relevance of emotional processes for RA while highlighting the differential contribution of adolescents’ attachment to mothers vs. fathers upon PA. Implications are discussed, and suggestions for future research are provided.