Well,a classic topic,and you've picked a good example to show duty is superior than love,because princess should take the responsibility both of royalty and country on her shoulder.And that's my opinion below:
"love is kind of a instinct.Just because of the purely good feeling so that we respect it,and we should admit that it's the most beautiful thing ever.And in fact,under most circumstances love & duty is not so contradictory,but sometimes when the predicament of "a choice between two" emerge in your life,we still should take duty as top-drawer,cause there are a lot of things in our life over the individual feelings.Duty ask for self-command,to be a conscientious man maybe not so pleasure as the self-heart follower,but that's the right thing to do,and that's the proof that prove you are a grown-up."