Why is my brother acting like this over a girl?

Well, before I start let me just say that my brother is in his senior year in highschool. He's my brother, but he's so handsome, jet black hair, dark blue eyes, tall, very muscular, jawline, long lashes, tanned. Suffice to say, girls not only in his school, but everyone who sees him says he's the best looking guy they've ever seen, and he's very popular with the girls in his school. He's doesn't have an ego though, and chooses to remain single because he wants to focus on school to pursue medicine. Anyways, we're really close. He told me about this girl in his school who's apparently gorgeous, but she has a terrible life. according to him, she never smiles, does every drug in the book, has an abusive family and slept around with the majority of guys in her school. He talks about her all the time, and says he doesn't know why, but he feels so attracted to her, and he's never spoke to her either. I've seen her on facebook, and she is pretty gorgeous, but i don't know why my brother is going so crazy for her. He can have any girl, why is he so attracted to a drug addicted, abused, promiscuous girl? I mean she's pretty, but He's ALWAYS talking about her. I'm not jealous or anything, I'm just frustrated that nothing comes out of his mouth besides" jessica did this, I feel bad for jessica, i wish jessica wouldn't do this, etc." Btw, he's 18, and i'm 15, but we go to different high schools.