Confused about my relationship?

I have a boyfriend, we've been together for over a year now. I love him...but I keep having dreams (both asleep and daydreams) about his best friend's brother. He isn't even my type, he smokes weed and dresses really badly and sits on his xbox for all his spare time. But I can't stop thinking about him. We've only ever talked once over facebook, after he saw me crying in the locker room after getting targeted by some bullies, when he told me if it happened again he could sort it out for me because he was a prefect. He was kind to me despite being almost a total stranger, just another face in the corridor. Since then he's been diagnosed with schizophrenia.

Why is this happening? How can I love my boyfriend but be thinking of someone else so much, someone I normally wouldn't find attractive at all?