You look sad when you think he can't see you
This one sentence irritated Sherlock more than he would admit.
"You look sad when you think he can't see you," With ' he ' Molly meant Dr. John H. Watson, his flat-mate - colleague and friend.
Sherlock had never thought, that there would be ever someone he would call a friend. There were people he tolerated and people he liked, but he would never call them friends. He tolerated Detective Inspector Lestrade and he liked Mrs. Hudson, but with John, it was something different.
It was strange. Since John moved in with him, he had integrated himself perfectly into Sherlock's everyday life. When Mike had asked him back then, if it wouldn't be easier if he had a flat-mate, he had asked him who would ever want him as a flat-mate? Who would put up with him? Sherlock knew that it wasn't simple to live with him, but John didn't seem to have a problem with that. Of course sometimes he ranted about things Sherlock did, but still. They were a perfect match. Sherlock couldn't imagine a life without John at his side. When did that happen?
Sherlock looked over at John, who was busy with analyzing a sample from the crime scene. He appeared to be completely focused and didn't seem to be aware of his surroundings. But Sherlock knew that impression was wrong. John noticed everything around him, something he couldn't set aside after his life as a soldier. John had rescued him on their first case together and he defended him in front of Anderson and Donovan, although they hadn't known each other that well at that point. The doctor trusted him unconditionally even though John's therapist believed he had trust issues.
There it was again. This streak of sadness, he was so aware of now, since Molly had pointed it out. Sherlock was worried. He knew about the utter loyalty of his friend and he knew about the danger. Moriarty once used John to threaten him. And John had risked his life to save Sherlock.
What if this time, they didn't have such good luck? What if Moriarty killed John this time? With this thought, Sherlock sensed a hurtful pang in his heart. He remembered that Mycroft once told him that 'caring is not an advantage'. Sherlock would once have agreed, but now John was in his life and everything had changed. Caring was an advantage. He would do everything to protect his friend John. Even if it killed him. John was more than a friend. Sherlock was – thanks to Molly – absolutely clear about that now.
"You look sad when you think he can't see you."
Sherlock conceived a grim plan. He would protect John until he had defeated Moriarty. And now he knew how he would do it...