Humans work to improve the situation of others through sympathetic concern. The empathic joy hypothesis proposes that the urge to help is stimulated by enhanced sensitivity to vicarious joy achieved through helping a recipient to meet their needs. We further hypothesized that the positive feeling-related brain activation that occurs in the striatum during sympathetic concern is enhanced by empathy in a familiarity-dependent manner. To test this, we conducted behavioral and functional magnetic resonance imaging experiments in which two participants simultaneously played a virtual ball-toss game where either the subjects' partner or an unfamiliar player was isolated by the other players. The subjects showed sympathetic behavior by tossing the ball to the isolated player and reported enhancement of self-positive feelings and anticipation of feeling improvements of the isolated player during sympathetic behaviors. Activation in the bilateral dorsal striatum became more prominent during the behaviors, supporting the empathic joy hypothesis. Dorsal striatal activity when helping a romantic partner positively correlated with affective empathic traits toward that partner, whereas the activity measured when helping a stranger positively correlated with perspective-taking traits. The correlation results further revealed a familiarity-dependent empathic enhancement of positive feelings. Therefore, sympathetic concern is invoked through empathic positive feeling.