Sherlock had no time for emotions. They were irrational – pointless. They were the result of a complex mess of connections in the human mind, developed simply to help the species survive in times when groups needed to work as a unit. Anthropologically – scientifically – he knew that emotions were an incredible feat of nature. He could appreciate how amazing the chemical reactions that occurred in the body to produce them were, in the same way that he appreciated the complexity of the human eye. The difference was that he needed his eyes to see, because seeing was what he did. But he didn't need emotion. In fact emotion was detrimental, he had concluded after much thought. It stopped him being able to see so easily – it blinded him. Emotion for him, he had concluded, was like a runner with rickets; it struck weakness right into the core of what he did best and it disabled him. He had felt it when the children at school called him things… And he couldn't focus in chemistry. And then, later, when his mother had died he had struggled to think for weeks. He couldn't even do simple mathematics. In a different time and place emotion might have been useful, but then and there in his life-time it wasn't; it was his mind that he needed to be functioning fully, and it only made sense to eliminate anything that affected it in a negative way.
Unfortunately, it was not as simple as cutting it out or pushing it to one side. The feelings always seemed to creep back in with the most stubborn tenacity, and sometimes it required something a little more to get rid of them.
Self-harm, he knew, was often seen as indicative of mental illness like depression or dissociative disorder, and that is why it was so important that no one ever found out about it. He knew that other people – stupid people – jumped to stupid conclusions after seeing something like that. They wouldn't see the real reason that it was important that he did it, and they would probably even try to make him stop.
It was out of carefully constructed routines that Sherlock was never discovered. He controlled all aspects of the process; made it methodical. The planning was just as important as the action itself, and the requirement to self-harm with no possibility of anyone finding out was a puzzle that his brilliant mind could focus upon. And just as much as he enjoyed the rush of endorphins from the pain and the perfect eradication of feeling, he also loved the sense of achievement he experienced when he had completed the methodical cycle with success. In that moment he felt perfectly in control, safe in the comforting ability of his own mind. No one ever found out, because Sherlock always did it perfectly.
When he got to university he was exposed to all kinds of street drugs that gave him the opportunity to experiment with alternate methods of numbing himself. He quickly settled upon cocaine and became a frequent user, and for a while the drug even replaced self-harm. But unfortunately cocaine was a lot harder for him to hide than his previous methods. Street rats that worked for his brother reported his substance misuse and Mycroft got… involved. Cocaine use came to an abrupt stop. For a while, anyway. But his primary coping mechanism did not come to an end; no one ever found out about the scars and cuts and blistering burns that decorated his skin. He returned to it with full intention when cocaine was taken from him. He wasn't sure if it was out of anger that Mycroft had intervened or anger at himself for losing control, but either way the following few months involved several occasions where he had to stitch himself back together.
It began to slow down slightly when he started doing consulting detective work for the police. When his mind was busy working on a case he felt good. High. High because he was taking control from someone who was harming people onto himself. And in that moment when he solved a case, finally, the rush of euphoria was so much better than pain or coke because he was then fully in control. In control of the criminal, the victim and the police. And also in control of himself, his mess of emotions: he feels nothing through the high.
And then it's just a waiting game for his next hit.