I'd say it's for the same reason you want to help them. Because they like you, they want to be your friend, and some see parts of you in them, too.

But ... sometimes that can be scary to face. When you realize how similar you are to someone you want to help, but it's something that you're scared to face, to accept or embrace. And then you realize that contradiction in yourself, and pull away. They might ask themselves that, too, 'why does somebody want to help me specifically'? And then that turns into an opposing force, where you want to help somebody that you know all too well, but would back away if the tables were turned.