let’s just consider how in the line ‘surrender becomes power’, the Joker is being extremely vulnerable–he’s essentially telling Harley that by submitting to him she’s going to have a leg up on him, that she matters enough to have some semblance of control over him that he’s willing to offer. Like he still needs that doctor part of Harley, to keep him at bay, that her presence as his grants her power to restrain him to an extent. He has something to lose. He has a stake. It’s said like he’s trying to be intimidating but for someone who’s trying to understand how to express emotions and comprehend love, it sounds a lot like ‘i need you’