Why am I attracted to him?

That's the difficulty of being gay/bi. You're going to fall for people that most likely won't reciprocate. And when you are forced to stay in the closet, you can't take a chance unless they are returning some really strong signals. As much as we can wish to choose who we fall for, we can't. I wish I could tell you something that would help. Just know that you are not the only one going through this.