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. Abstract: File sharing in clouds means that users store and share their files in clouds, which can greatly improve the utilization rate of the files. Traditionally, users in clouds are assumed to be willing to store and share their files in clouds, where the security and efficiency issues are discussed. However, little works involve the incentives for users to share files in clouds. That is, why user are willing 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. The main task of this paper is to find proper strategies such that users in clouds have incentives to share their files with others. To complete this task, 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. That is, the rounds needed for users to adopt cooperation when they take WSLS are fewer than those when they take TFT. Furthermore, WSLS is robust for unintentional deviation and returns to mutual cooperation after deviation. However, TFT is not robust for unintentional deviation. Thus WSLS is better than TFT for users to share files in clouds.Keywords: File sharing, Win stay lose shift strategy, Game theory, Cloud computing. Qiuliang Xu is a Professor and doctoral supervisor of computer science and technology at Shandong University. Currently he is a sub-decanal of department of science and technology at Shandong University. He has long-term research on theoretical study of information security, cryptology etc. He has published more than 30 high-quality papers in the most famous academic publications in recent five years. Now he is a chair of Nation Science Foundation of China and other foundations.