This case study of Slow Food at the University of Wisconsin-Madison (SFUW) examines the success of a volunteer social innovation that promotes sustainable food access. Using inductive thematic analysis of 19 in-depth, semi-structured interviews of past and current members, advisors, and collaborators, we find that the success of SFUW is consistent with Self-Determination Theory. We find that among the participants, the organization satisfies the basic psychological needs for autonomy, competence, and relatedness, which in turn support intrinsic motivation. In this way, volunteering tasks become interesting, fun, and are more likely to be performed over time. The findings suggest that needs fulfillment and intrinsic motivation are linked to creating a self-propagating social innovation where volunteers generate new ideas and successfully transfer skills to others.