This study discusses how players make the mutually cooperative mechanism in Social Games. In SNG, many players exchange helps each other. That mechanism is not only one-off but also repetitive process. In the perspective of reciprocity in Evolutionary psychology and Mythology, this study analyzes mutual cooperation in several game texts most well known in the SNG field. According to the field study results, four cooperative mechanisms were extracted. These 4 principles apply to every digital game design for emerging of cooperative storytelling among players.