1. A computer-implemented method for developing a profile of a user, the method comprising: determining digital content items interacted upon by the user, each of the digital content items associated with a plurality of topics; determining the plurality of associated topics for each of the digital content items; retrieving access data indicating the user's interactions with the digital content items; selecting profile topics based on the topics associated with the digital content items and the retrieved access data; storing the selected profile topics in association with the user's profile; determining additional topics related to the stored profile topics, the additional topics co-occurring with profile topics in additional user profiles; and storing the additional topics in association with the user's profile.