1) Abnormally preoccupied 2) Addict 3) Admirer 4) Aficionado 5) Besotted 6) Buff 7) Compulsive, fixated 8) Dedicated 9) Devoted 10) Devotee 11) Enthusiast 12) Extremist 13) Fervent 14) Fixated 15) Follower 16) Habitual 17) Irrational 18) Keen 19) Monomaniacal 20) Neurotic 21) Obsessional 22) Over-enthusiastic 1) Anal 2) Enthusiastic 3) Fervent 4) Inspired 5) Obsessional 6) Psychoneurotic 7) Stickler 
obsessive 1. A reference to or characterized by obsession. 2. Relating to, characteristic of, or causing an obsession: 'She had an obsessive-gambling habit.' 3. Excessive in degree or nature: 'The coach had an obsessive need for his team to win.'
Obsessive thoughts are ideas, pictures of thoughts or impulses, which repeatedly enter the mind.
noun a person who has obsessions