App-based carpooling services such as GrabCar Bareng offer potential for economic efficiency and environmental sustainability, but the adoption rate in Indonesia is still low. This study aims to explore the reasons why non-users refuse this service, even if they are familiar with ridesharing. Using a qualitative approach and thematic analysis based on interviews with 32 informants, this study identified three main reasons for adoption, namely economic benefits, environmental benefits, and social benefits, as well as six main reasons for rejection, namely time risk, social risk, privacy risk, comfort risk, security risk, and psychological risk. In addition, four situational and personal factors  novelty seeking, altruistic value, time pressure, and preference for social contact were found to influence the power of these reasons. This research expands the application of Behavioral Reasoning Theory in the context of digital transportation services and provides practical implications for service developers to design communication strategies and service features that are responsive to user concerns.