Voluntary engagement is an important prerequisite for the production of club goods. Although unpaid, the individual decision for or against voluntary engagement can be regarded and formally modeled as a deliberate act of social exchange using elements of behavioral economics. We lay out a simple behavioral model that captures in a stylized way several motives (consumption of the club good, social recognition, human capital, etc.) that may explain why individuals volunteer. We then use results from an interview study to assess the quantitative importance of the different motives, and to shed light on dimensions along which the model can be extended in future research.