As social media grow in popularity, more and more people produce user-generated content (UGC) in various types of social media. Thus, practitioners are interested in how to support people’s motivation to produce UGC. While previous literature has examined various factors influencing people’s motivation to produce UGC, few studies have examined the role of other participants as well as the social media sites. Based on self-determination theory, our study examines the relationship between content attractiveness, trust, perceived competence and autonomy, individual differences, and motivation to produce UGC. Using social network sites as the context of our study, we tested our hypotheses with U.S. college students, and the results supported our hypotheses. We contribute to the current literature by demonstrating that, when individuals perceive that other participants enjoy their UGC and trust social media sites, the individuals’ perceived competence and autonomy are enhanced, which in turn supports their motivation to produce UGC.