This article uses job demands-resources theory to build a model of public service motivation (PSM). Public service motivation determines how employees in the public sector deal with their daily job demands and resources. Highly motivated public servants are able to deal with their job demands and prevent exhaustion. Additionally, because of their sense of calling, they are motivated to mobilize their job resources to stay engaged and perform well. However, if job demands are consistently high and job resources are consistently low, highly motivated public servants will lose their psychological resources, resulting in lower PSM. Reduced PSM, as a consequence, may strengthen the loss cycle of job demands and exhaustion and weaken the gain cycle of job resources and engagement. Public service managers and employees may use this model to optimize their work environment on a day-to-day basis.