For many Internet consumers, live streaming has replaced traditional media outlets and become the preferred channel for information acquisition, entertainment and motivation of online expenditures. Maintaining users' loyalty and motivating them to be actively involved in monetization activities are some of the most critical challenges for live streaming platforms' sustainable development. To deal with these challenges, our research aims to investigate user engagement behaviors in the Pay-What-You-Want (PWYW) context and the underlying mechanisms behind these behaviors in live streaming platform. By collaborating with a popular live streaming platform (Remix) in China, we combine a unique panel data analysis method on minute-level user activity data with large-scale field experiments. In our live streaming context, we analyze users' paying (tipping) behavior through manipulating audience size, tipping amount and examining the role of emotion in interactive and dynamic social settings. Our findings can help live streaming broadcasters and platforms better understand user engagement behaviors, and thus provide theoretical and managerial implications for effective marketing strategies to enhance users' loyalty and stimulate their active engagement.