Disclaimer: I don't own Sherlock or any of these characters.
So many parts of Sherlock's life changed when he met John Watson. His eating habits, social skills, thinking process, all changed. The biggest change to Sherlock though was to his mind palace. He remade his mind palace so every place in it would remind him of John. He had rooms upon rooms of information about his flat mate, all of it he considered important. He knew how John liked his coffee and his moods from the height of his eyebrows. Sherlock knew how John's day had been by how his boots were tied and which tea to have ready when John came home by his use of punctuation. He had rooms dedicated to noticing how perfectly he and John fit together. The heart and the brain, one always lost without the other. John, the man with trust issues who killed for Sherlock on the first day they met, John who could put up with him and even enjoyed it sometimes. John, who called him brilliant and extraordinary, John who was underestimated by so many yet understood by few. John, whose hands had healed many but killed as well, wonderful, mysterious, brilliant, his John. Sherlock could not understand what made John stay when so many others had ran. He had been told so often that he didn't have a heart that he started to believe it until he met John. John was his heart, his moral compass, his guardian angel. Sherlock knew so much about John, yet it never seemed enough. The seemingly ordinary army doctor kept surprising him, giving him new information every day until Sherlock was worried he wouldn't be able to hold it all. Sherlock, who deemed the solar system useless, refused to delete any tiny detail about John. To Sherlock, John was more important than the stars.