Watching online videos on mobile devices has been massively present in people's daily activities. However, different users can watch the same video for different purposes, and hence develop different expectations for their experience. Understanding people's motivations for watching videos on mobile can help address this problem by giving designers the information needed to craft the whole watching journey better adapting to user's expectations. To obtain this understanding, a comprehensive framework of viewer motivations is necessary. We present research that provides several contributions to understanding mobile video watchers: a thorough framework of user motivations to watch videos on mobile devices, a detailed procedure for collecting and categorizing these motivations, a set of challenges that viewers experience to address each motivation, insights on usage of mobile and non-mobile devices, and design recommendations for video sharing systems.