My mother-in-law owned a lhasa apso years ago and he seemed very protective of her in the same way you are describing your dog. One time when she was sitting in a chair and he was on her lap I leaned over to hug her and he snapped at me as if to protect her. So I believe it is in their nature to be like that. Your Charlie is really cute. Thank you for rescuing him and giving him the best life possible. All animals deserve that.