Using repeated cross-sectional survey data for private-sector workers in metropolitan Japan (2011–2024), we examine how job and life satisfaction relate to union demand and supply, distinguishing realized membership from frustrated demand. Frustrated demand is associated with lower job and life satisfaction only where no on-site union is present; where one is present, workers with frustrated demand are about as satisfied as those with no interest in membership, indicating that it is the mismatch between demand and supply, not unmet demand per se, that is costly. This pattern is sharpest among standard (seishain) employees, whereas among non-standard employees union presence does little to reduce frustrated demand, consistent with limited eligibility. We find that union presence is associated with higher satisfaction among non-members who have no interest in joining only if they regard unions as necessary, consistent with spillovers that create scope for free-riding, while the lower satisfaction of members who do not think unions are necessary is unrelated to union presence. The wellbeing consequences of unionization in Japan thus depend not on demand or supply alone, but on whether the two are matched.