Sometimes, Sherlock just needed an escape. A break from the dizzying effects of his intellectual genius. When he was younger, he got that escape from the drugs he forced upon his abused body. Cocaine, his weapon of choice in the war against his own mind. His reaction to the drugs was amazing, the effects they had astounding, stimulating his brain like nothing else, he felt the euphoria of a high in the clarity of the world. Under the influence, he could see things that even he would normally miss. But before long, he became dependent, addicted, to the drugs, or more specifically, the adrenalin rush he got. It went to his head, affecting his ability to deduce, his brilliant mind hindered now, until even forming coherent thoughts felt like wading through thick honey, the sickly sweet smell cloying his mind, the sticky, viscous liquid clogging every vessel. It took a while, but with the help he received from an unlikely source, he was able to get himself back on track. Those few years were a definite low point for Sherlock; a darkness, shadowing days gone by with its overcast presence, that, although he did not want to be reminded of, he could not bring himself to delete. His past had shaped him, moulding him into the man he was now.
Sherlock Holmes, consulting detective, self-proclaimed high-functioning sociopath, genius. He was not a hero. He was not a good man, but he could become one, with the help of his friends. He had told himself he didn't need friends, he didn't have friends. "I've just got one," he had said. Since then, he had come to realise that the science of friendship wasn't that simple. Moriarty had taunted him with the threat of his friends' deaths - John, obviously, but Lestrade and Mrs Hudson too. The term 'friend' had now encompassed Molly, as well, since she had proved her worth in her unfailing loyalty and invaluable assistance and support with Lazarus. His perspective of the universe had changed indefinitely throughout the years he had spent with John as his flatmate, and the subsequent years of solitary loneliness. He had had to re-evaluate himself and the world he found himself living in.
Sherlock had had no idea how much pain his heart could hold, until he had past the point of no return, until he had had to give up everything in his life that made him happy. Everything he thought he knew about himself had been ripped apart. His very soul had been exposed for the eyes of Torment, Guilt, Fear and Anguish to destroy, then re-moulded into a version if himself that was a stranger to his body and mind. He was not the same man that had walked out into the sunlight on that roof, that fateful day when everything changed.
Ever since 'The Reichenbach Fall', as it had been christened, he had hidden away, not just physically but mentally as well. He had withdrawn into himself, imprisoned himself in his mind palace with only his subconscious operating on a physical level. Throughout the years since his drug habit had receded, he had built up his mind palace to become his sanctuary. A place only he could access, where he could be alone, wherever his body was, whoever he was with. It was his sorting office, storing any useful data on his hard-drive; deleting unnecessary information to stop it clouding his clarity when it was vital. It was the place he retreated to when he needed to think, or just some time to himself. Sherlock's mind palace was his coping mechanism, and it took whatever form his subconscious deemed necessary for his need. Depending on his mood, he could end up anywhere.
Seeing John at his 'grave', hearing his broken voice, had crushed Sherlock. All the blocked emotions exploded as the dam burst, flooding his mind and short-circuiting his brain. Standing there, under the shade of the tree, watching John's retreating form, small and sad, he felt completely drained, of energy, of emotion, of life.