Watson: Is that why you've been writing him? Because you want to... understand him?
Moriarty: Part of it, I suppose.
Watson: And the rest?
Moriarty: What do you think?
Watson: You think you're in love with him. Only you can't be sure, because as much as you claim to know about the world, love is something you don't quite get.
Moriarty: You'd be surprised what I'd do for love.
Watson: Nothing crazy people do surprises me.
Moriarty: I write to Sherlock because he's the only person on the planet I can really talk to. He writes to me because I'm the only one he can talk to. The only one he can ever truly relate to. If you still don't understand that, you will someday. I promise.