Obsession

noun: obsession

    the state of being obsessed with someone or something.
    "she cared for him with a devotion bordering on obsession"
        an idea or thought that continually preoccupies or intrudes on a person's mind.
        plural noun: obsessions
        "he was in the grip of an obsession he was powerless to resist"
        synonyms: fixation, ruling/consuming passion, passion, mania, idée fixe, compulsion, preoccupation, enthusiasm, infatuation, addiction, fetish, craze, hobby horse;