Despite a rising concern over this emerging global issue, theoretical understanding of the excessive use of massively multiplayer online games (MMOGs) is lacking in the mainstream IS literature. In this study, we proposed a research model to theoretically explain the development of the excessive use of MMOGs with the positive-negative reinforcement perspective. We tested our research model with 513 active users of MMOGs. Our results suggested that both positive reinforcement (i.e., perceived enjoyment and the associated positive affect) and negative reinforcement (i.e., withdrawal and the associated negative affect) have strong influence over the development of the excessive use of MMOGs. This study provided a theoretical explanation of the excessive use of MMOGs and implications to researchers and practitioners.