The fantasy of being rescued isn't a reality, the person who wishes this is more than capable of rescuing themselves and in fact in psychotherapy that is the desired outcome. The desire comes from a learned helplessness, that people will swoop in and do the uncomfortable work for them, it allows them to stay in a childlike place for longer. Sometimes it is sheer manipulation of wish for attention or proof that someone really cares about them, a test if you like. Or sometimes it's a passive aggressive way of getting needs met.