The doctor is in!
John and Sherlock getting into an argument after Sherlock rudely deduces someone and John hisses bitterly, "how do I know you even love me, Sherlock? You got engaged to break into an office! Is that why you proposed to me, hmm? Something I have that you want?" and after a moment of silence, Sherlock nods sheepishly. "What, then? What do I have that you want?" and Sherlock finally looks him in the eye and says "your surname."