Abstract Earthquakes are notoriously irregular, yet occasionally neighboring faults rupture in close succession, repeatedly, as if synchronized. Over successive cycles, differences in recurrence intervals, overall irregularity and external stressors should all drive neighboring faults out of phase. We propose that synchronization emerges only when fault interaction is phase dependent and exceeds the accumulated misalignment. The competing effects are quantifiable from the geometry of neighboring faults, yielding a conditional existence prediction for synchronization that we test first against simulations and then observations. Among repeating earthquake families in the central San Andreas Fault, the overall prevalence of synchronized pairs decays rapidly with increasing separation, tracking their stress transfer. Among neighboring megathrust segments in the 1000‐year historical record for Japan, the same threshold identifies segments in Nankai and Hokkaido as uniquely poised for synchronization. This work offers a mechanistic basis for earthquake synchronization and, more broadly, shows how interaction shapes earthquake recurrence.