Despite strong policy support for volunteerism, Chinese social service organizations require assistance in volunteer management, particularly concerning retaining volunteers and sustaining the supply of volunteer services. By interviewing volunteers from a successful volunteer organization and analyzing the collected data using the constructive grounded theories and methods, this study found that, contrary to the conclusions of previous studies, (1) sustainable volunteerism involves volunteers’ ability, motivation, and resources; (2) the ideal framework for the sustainability of volunteer service provision is based on a triangular support model of “individual-organization social interaction”; and (3) social interaction manifests itself in three network effects (emotional, family, and social). In particular, our analysis found that interpersonal relationships, intergenerational relationships, and parent–child education are essential to sustaining the supply of volunteer services.