Everyone who met Sherlock knew that he was closed off. Within minutes of meeting him, people could tell that he built his walls up high and didn't let anyone in. They could tell that he was emotionally distant, private person. What they didn't realize, was that Watson was this way too. In the weeks that he had known her, Watson had revealed very little about herself. She hadn't told him anything about herself and what little he had learned about her was superficial and inconsequential. He wanted to know what made Joan Watson tick, and it infuriated him that he didn't. Why couldn't he figure her out? He supposed that it was the same thing that tricked people into thinking that Watson was warm, open and emotional, when really, she was more emotionally closed off than Sherlock himself. It was her size and that youthful quality that she had that most likely threw people off. She just seemed so young and sweet, she seemed relatable which tricked people into thinking that they were seeing every part of her, when really what they saw was only a tiny portion of herself that she chose to reveal to them. Sherlock would have laughed at their stupidity if he wasn't part of the mass of people that couldn't figure out Joan Watson. She was quite the little con artist, tricking everyone into thinking that she was this vulnerable little thing, when really she had this inner strength that was masked by her seeming innocence. He was sure that she genuinely cared about people, what he wasn't sure of was to what degree. She might not be as damaged as he was, but she was definitely much more damaged than she let on. Yes, she would help him get over his addiction, but not quite in the way she expected to, instead of overcoming his "triggers" by talking about his feelings, he would supplement his addiction to drugs with his addiction to solving puzzles. Human puzzles were his favorite, and he couldn't wait to crack the code that was Joan Watson.
Irene obviously means a lot to Sherlock. For a man who was trying to conceal his damage, he wasn't controlling his reactions very well. What he doesn't realize is that his freakout told Watson more about him than an honest answer would have. Being a doctor, she'd learned how to figure things out about a patient even when they wouldn't tell her anything. Especially when they wouldn't tell her anything. Sometimes, the way a person reacts to a question tells you much more than their answer would have. He thought that he was so talented at concealing his emotions, but the truth was that he had no idea. When Joan's older sister had gotten heavily into drugs and promiscuity and eventually died because of the HIV that she had contracted due to her lifestyle while Joan was still in high school, Joan hadn't shed a tear. Instead, she had vowed that she would become valedictorian in high school and college so that she could become a doctor and find a cure for the disease that had killed her sister. She knew that this aspiration was more than a little conceited and grandiose, especially because since then, doctors had found a treatment for certain strains of HIV, but still no definitive cure, so Joan had stayed on her crusade against HIV, but when she killed a patient on the operating table, she thought that medicine might not be the best way for her to prevent other people from falling prey to the same disease that killed her sister, so she decided to help addicts get back on their feet, to keep them from backsliding into a downward spiral that would eventually lead to their death. No one knew why Joan had really become a doctor. They thought that it was just her ambition and her desire for upward mobility that had driven Joan to become a doctor, and that she had suffered a crisis of conscious after killing her patient and that she had become a sober buddy because she had lost her way. Very few people knew anything about her, and she liked to keep it that way.
It was an odd little dance that they had, while Watson asked questions about his personal life, he would try to figure out what was going on in hers. While he tried to figure out the best way to trick her and stop her from finding out who he really was,Watson tried to learn about the inner workings of his mind, supposedly to help him recover, but Sherlock could tell that it was more than that. He could see in Watson that same hunger for knowledge, that same desire to learn everything about other people while revealing nothing about herself. In a way, she was kind of his kindred spirit. Her deductive skills were surprisingly sharp for a person who hadn't spent years honing them. He enjoyed her intelligence that the knowledge that he would always be the smartest person in the room, except, maybe, when he was in a room with her. He couldn't let himself get emotionally vulnerable with her. After all, it was his feelings for Irene that had caused him to become addicted to drugs and hit rock bottom in the first place. She had found the cracks in his otherwise impenetrable emotional armor and wormed herself into them. She opened them wider and wider until his armor cracked completely and he was left naked in front of her. Once she had exposed his every vulnerability, exploited his weaknesses, made him fall in love with her, she had left him, emotionally strung-out in the streets of London, a broken man left to fend for himself. He couldn't stand hurting so much so he decided to drink to numb the pain, when that didn't work and made him more morose than before, he decided to move on to stronger substances. He had heard that a single dose of heroin could effectively numb any pain, no matter how heart-crushing. For a few blissful hours, he would forget all about Irene and how weak he had become. He told himself that it would just be a few hits to distract him from Irene and help rebuild his armor, but it wasn't. Soon, he started to lose interest in his cases. After he had gotten fired from Scotland Yard, he had ended up on his friend's couch, so drugged up that he couldn't even form coherent words, except for one, Irene he had been told that he moaned her name for hours. It was pathetic. He was an emotional mess, left to pick up the broken pieces of himself and his emotional armor. It had taken him months to come back to some shattered shadow of who he used to be. He had gone to rehab, he had followed the program while revealing as little about himself as possible. The last step of his recovery that would require him to pretend to reveal personal details about himself would be his six weeks with his "sober buddy" or his "addict sitter" as he preferred to call her. As long as he made sure that he kept his emotions hidden, he wouldn't lose himself again. He didn't know what it was about American women that made him lose all of his inhibitions, but once he figured it out, he would make sure that he would never let it affect him ever again.
Joan couldn't believe that she was starting to feel more than just intrigue towards Sherlock. She knew that he was damaged, narcissistic, and that he had an extremely overinflated sense of self, yet somehow, she was starting to feel genuine affection towards the abrasive misanthrope. She had no idea why, in the past, the people she had liked had always been kind, gentle, and most of all, simple, not self-obsessed, damaged and complicated. Maybe it was because she sensed something in him, something she recognized in herself, some dark part of her that even she didn't want to admit existed. In the past she had tricked herself into thinking that she was all smiles and sunshine, but maybe she wasn't, maybe the inside of her was dark and twisted and damaged, and for the first time, she was starting to think that maybe that was okay. She might not be able to help Sherlock heal, or even know who he really was, accept what she knew about him because she knew that there was the same darkness inside of her, and it was that darkness that made them the talented people they were. Unfortunately, it was that immense talent and hunger for knowledge, that insatiable need to know everything about the inner psyches of the people around them that drove other people away and left them completely alone except for each other, but maybe they were all that the other person needed.