It is incredibly amusing to watch the circus of emotions jump across Castle's face. It's like he can't believe she actually asked the question, when only moments ago he'd theoretically extended the invitation. He feels his stomach contract around a ball of theoretical possibilities.

"No. I mean, yes. I mean, I've been thinking about it. You know." Suddenly Castle seems overcome by a case of the sentence fragments. He bites down on his back molars and works to throw some conjunctions in there, looking at the front of the coffee maker like he could burn holes through it with his retinas. "I mean, in my head it makes sense. You already keep a toothbrush here and I'm technically closer to the precinct than your apartment."

Those are his arguments, as he sees them: cavity prevention, and a shorter commute.