I think it’s a combination of basic physical attraction and that she does appear to be an ideal portrait of innocence. He has her on a pedestal, so I’m not sure how well he really knows her, but he has shown he’s not without insight so he probably knew her better than anyone else in KL at the time. But the thing is, it’s back to the same idea- she is what Sandor can’t have- peace, safety, innocence, goodness. Or what he thinks he can’t have.