It is argued that there are interconnected and dynamic factors that influence the sustainability of water, sanitation, and hygiene (WASH) services in developing countries. Five sustainability factors are often mentioned by literature: financial, institutional, environmental, technical, and social (FIETS) factors. This article aims to understand the interaction of those five sustainability factors in the context of WASH services in Indonesia. A system approach, in the form of a “modified” causal loop diagram, was utilized to analyze this interaction. The strength of influence from one factor to others were obtained from small surveys of 16 WASH practitioners and experts from international and national non-governmental organizations (NGOs) based in Indonesia and national government institutions related to WASH. It was found that institutional factor is the “leverage” factor, i.e., the most influential and least dependent, in the WASH services system in Indonesia. It means that WASH institutional strengthening is necessary to sustain WASH services in Indonesia. Furthermore, the most important feedback loop is social–WASH services. This depicts the importance of the bi-directional effect of WASH services and the social conditions of the community, i.e., the more supportive the community is, the higher the likelihood of the sustainability of WASH services, and then positively influence back the social conditions of the community. Moreover, the relationship between factors varies across context or location. Finally, this paper shows that the sustainability of WASH services involves complex interactions of the sustainability factors and understanding these interactions is critical to sustaining the WASH services or program in rural Indonesia.