Sherlock was still awake. It was three in the morning, but he hadn't moved from his seat since John had gone home with Mary at half past one. They had been up hunting down a cat burglar who had been steadily increasing the value of his thefts and completely baffling the police. As usual, it had been incredibly simple. That wasn't what had Sherlock awake.

John and Mary had been married for about five months now. Despite Sherlock's original concerns (which he would never had admitted) that John would leave after marrying, the two still worked together often. Mary came along sometimes, her experience was occasionally even useful.

So not much had changed and yet Sherlock's world had been shaken. He had seen changes in John's behavior since he'd met Mary and Mary had changed entirely because of John. Sherlock had always stood firm in the belief that love and any other sort of affection was irrelevant. Actually less than irrelevant, it was a weakness. And marriage didn't even serve a purpose really. It didn't affect anything.

And yet, John was faster and more confident when he made a conclusion since the marriage. Mary had been a stone cold operative, but now would rather arrest someone than kill them. Why?

Sherlock folded his hands together and pressed his fingertips against his chin. He closed his eyes. Opening them, he ran through his mind palace and stopped at a door. He opened it and stepped inside. Ms. Hudson was telling him about how marriage changes people. She was kind of rambling though, so he turned around and left.

He stopped at a different door and entered. Molly was there with a Christmas present for him. He looked at her for a moment. She answered his unspoken question.

"Sherlock, I keep helping you because I love you. You know that."

He sighed. "Love or affection of any sort is just a chemical reflex prompted by appreciation of some trait in a person. It means nothing, changes nothing. So what is causing this change in behavior in both Mary and John?"

Mycroft answered. "It doesn't matter, you know. Affection of any sort is just human error. Irene Adler for example. She would have succeeded, but for her fondness of you."

"Yes." Sherlock mused. "Which I returned when I saved her."

"Don't get too attached, brother mine." Mycroft warned. "Remember Redbeard."

Sherlock left the room. He wandered up his staircase to another room. John was there, sitting in their common room and frowning at the wall with the smiley face target. He turned to Sherlock as he entered.

"You drive me mad, Sherlock. You really do. I'll still keep coming back to help you though. You're my best friend."

Sherlock sat in his seat across from John. "Your friend. John, I don't have friends. Any sort of affection is a weakness. I have none."

Moriarty appeared. "Oh come on, Sherlock. We both know that's not actually true."

"I'm not discussing this with you right now." Sherlock dismissed him.

"He does have a point though, Sherlock." John commented. "If you really are heartless, why did you let the Black Lotus assassin escape and save us instead? Or run into a bonfire to save me? You put yourself in danger from Moriarty to keep all of us from being shot. Over and over, you sacrifice something for someone else despite your claims of not caring."

"You're different, John. You're more of a brother to me than Mycroft ever was. You're still an idiot, but less so than everyone else. I can put up with you."

"There, Sherlock. That right there is you admitting your caring about me changing you. Romantic love is similar and marriage is just the confirming of it." John sighed. "You still don't understand, do you?"

Sherlock took a deep breath. "No. I don't."

Mary appeared. "Sherlock, look at it this way. Love isn't a chemical reaction. Sentiment, maybe, but love is simply choosing to continue to stand by each other no matter what. And marriage is the promise."

"No, I still don't get it." Sherlock stood. "You're all just so sentimental and there's no advantage to it."

Sherlock opened his eyes. He got up from the couch and looked at the clock. It was now a quarter to six. John would be back soon. They had an appointment with a potential client at seven. Sherlock walked over to the refrigerator to get a drink.

He considered a nicotine patch, but discarded the idea. It wasn't really good for him and besides, he didn't want to disappoint John. Sherlock paused and considered that. Maybe there was some benefit to sentiment after all.