Traditionally, users in clouds are assumed to be willing to store and share files in clouds, where the security and efficiency issues are discussed. However, few studies involve the incentives to share files in clouds. In this paper, we delve into users' incentives for using the cloud system to store and share their files towards the view of game theory. More specifically, the process of file sharing is reduced to an infinitely repeated prisoner's dilemma PD game and the action of sharing files in clouds is considered as the action of cooperation in the infinitely repeated PD game. We incorporate win stay lose shift WSLS strategy into file sharing and simulate it compared with tit-for-tat TFT strategy in clouds. Simulation results show that WSLS is an optimal strategy for users to share their files in clouds. Furthermore, WSLS is robust for unintentional deviation and returns to mutual cooperation after deviation.