If you are excessively enthusiastic about something — a sports team, an actor, your religion, saving the whales, a certain brand of chocolate — then you are fanatical about it.
Fanatical comes from the word fanatic, which itself came from the Latin fanaticus, meaning "mad" or "inspired by a deity." The root word is fanum, or "temple." The original English meanings of both fanatic and fanatical implied that the person being described was insane with enthusiasm, like a religious zealot. The word fan is likely shortened from fanatic, and thus shares the same roots, but that word doesn't imply the same obsessiveness that fanatical does.