Alternatively maybe she's seen something of the future and there's a thing that she needs him specifically to do. Maybe he wasn't picked at age 16, maybe she's been watching him since birth, waiting for him to grow old enough to hold the power. Faeries being faeries it may be that the thing she needs him to do is something perfectly innocuous, it just has to be him specifically that does it, or it could be that she's using him to keep the power from being reassigned by her mother (maybe there's a special someone that the Queen want's to name asKnight, but the Lady doesn't like). At that point you've got the Knight being assigned really dangerous tasks by the Queen in an attempt to get him killed (because she can't be seen to do it herself, after all , he seems loyal to the court), and the Lady trying to keep him out of harms way by putting him in more low threat diplomatic situations (which may inevitably turn out to be more complicated than they appear).