How does emotion explain the relationship between social rejection and aggression? Rejection reliably damages mood, leaving individuals motivated to repair their negatively valenced affective state. Retaliatory aggression is often a pleasant experience. Rejected individuals may then harness revenge's associated positive affect to repair their mood. Across 6 studies (total N = 1,516), we tested the prediction that the rejection-aggression link is motivated by expected and actual mood repair. Further, we predicted that this mood repair would occur through the positive affect of retaliatory aggression. Supporting these predictions, naturally occurring (Studies 1 and 2) and experimentally manipulated (Studies 3 and 4) motives to repair mood via aggression moderated the rejection-aggression link. These effects were mediated by sadistic impulses toward finding aggression pleasant (Studies 2 and 4). Suggesting the occurrence of actual mood repair, rejected participants' affective states were equivalent to their accepted counterparts after an act of aggression (Studies 5 and 6). This mood repair occurred through a dynamic interplay between preaggression affect and aggression itself, and was driven by increases in positive affect (Studies 5 and 6). Together, these findings suggest that the rejection-aggression link is driven, in part, by the desire to return to affective homeostasis. Additionally, these findings implicate aggression's rewarding nature as an incentive for rejected individuals' violent tendencies. (PsycINFO Database Record