"Users come to YouTube for a wide variety of reasons which span a spectrum from more to less specific: To watch a single video that they found elsewhere (direct navigation), to find specific videos around a topic (search and goal-oriented browse), or to just be entertained by content that they find interesting. Personalized Video Recommendations are one way to address this last use case, which we dub unarticulated want."
YouTube states that "the goal of the system is to provide personalized recommendations that help users find high quality videos relevant to their interests. In order to keep users entertained and engaged, it is imperative that these recommendations are updated regularly and reflect a user’s recent activity on the site. They are also meant to highlight the broad spectrum of content that is available on the site."
"To compute personalized recommendations we combine the related videos association rules with a user’s personal activity on the site: This can include both videos that were watched (potentially beyond a certain threshold), as well as videos that were explicitly favorited, “liked”, rated, or added to playlists."
User site activity - videos watched, uploaded, rated, favourited
The recommendations feature has been part of the YouTube homepage for more than a year and has been very successful in context of our stated goals. For example, recommendations account for about 60% of all video clicks from the homepage.