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 emotional energy levels that are represented within a three-dimensional pleasure-arousal-dominance model of emotional states and are humanly perceivable in the playbacks of the media files; generating, by one or more processors of the machine, a distribution of the play counts over the arousal scores, the distribution pairing play counts to corresponding arousal scores, the distribution including one or more relative maximum of play counts that each correspond to a different arousal score among the arousal scores; detecting a biometric measurement of the user via a biometric sensor; determining, by one or more processors of the machine, a current arousal score of the user based on the biometric measurement of the user; 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; identifying, by one or more processors of the machine, a first relative maximum of the one or more relative maximum having a first arousal score closest to the target arousal score and based on the current arousal score of the user; and selecting, by one or more processors of the machine, a media file for playback to the user based on the media file having an arousal score at or near the first arousal score of the first relative maximum.