There isn't anything wrong with you. But, and I'm sorry to put it bluntly, this guy sounds like he's doing nothing more than objectifying you. Maybe he finds you physically attractive, maybe he likes the thrill of being noticed, maybe he's a little bored with his GF, or maybe it's all three. But it really isn't about you as a person, just the sexual nature of your interaction. I feel like doing what you can to avoid him and focus on your boyfriend, or even yourself, may be more healthy for you in the long run.