1. A method comprising: accessing, by one or more processors of a machine, play counts that quantify playbacks of media files played for a user, the play counts being accessed from a playback history database of the user; accessing, by one or more processors of the machine, arousal scores of the media files played for the user, the arousal scores indicating energy levels humanly perceivable in the playbacks of the media files; determining, by one or more processors of the machine, a distribution of the play counts over the arousal scores, the distribution indicating relative maximum play counts that each correspond to a different arousal score among the arousal scores; selecting, by one or more processors of the machine, a target arousal score for the user based on contextual data that describes an activity in which the user is engaged; and selecting, by one or more processors of the machine, a media file for playback to the user, the selecting of the media file being based on the target arousal score and based on an arousal score that corresponds to a relative maximum play count included in the distribution of the play counts over the arousal scores.