When Sherlock Holmes was five, he was introduced to people other than his family. Mycroft, who was twelve then, already shined on every important meeting, gladly talking about school, life and his personal achievements. Sherlock wasn't as cooperative as his brother and, being only five years old, he soon complained his way out of every official dinner possible. It wasn't much use keeping him there anyways, the only thing the boy ever did was repeating 'dull' over and over again or exposing lies that were made in purpose of obligatory flattery. That way, instead of wasting his time at the dinner table with boring adults and his older brother, he could master the art of reading, difficult for every five year old boy, no matter how brilliant.
When Sherlock was ten, he could no longer escape all the horribly dull dinners held at his house. He was a big boy now, going to school and shining like a little star, so he couldn't hide away in his room anymore. Sherlock tried to scream 'dull' once or twice, but Mycroft, being the perfect teenager that he was, silenced him in an instant. Sherlock got very angry at him, but later they made a deal- Sherlock would act like a normal, perfect little boy and later Mycroft gave him his cool books from school to read. And, to be quite honest, even without the Holmes' brothers' deal, dinners weren't the most problematic. What Sherlock hated most was school, his boring classmates and under qualified teachers. He was smarter than all of them and it was so tiring.
When Sherlock was fifteen, things haven't changed much. His life consisted of books, experiments and occasional insults (or objects) thrown at him. He didn't care much, though, for he was far better educated than they ever had a chance to be. Besides, it's not like he showed up at school every day. Once his teenage angst had set in, so did his rebellious attitude. Discouraged by the stupidity of the school staff and students, he mostly skipped class and examined birds and insects in the nearby park. If he got very lucky, he found a dead bird- they were a treat for the boy, art of decomposing at its finest. Sometimes he was brought to school almost by force- he spent his hours there sulking and mumbling insults at everybody. Except one physics teacher, who took the boy under his wing and showed him the wonders of science. It still wasn't enough to bring Sherlock to school, but the boy surely was grateful for the man's attention, even if he never expressed his gratitude.
When Sherlock was twenty, he was already in college. He could be studying whatever, it didn't matter- he already knew most of what any professor could tell him and even if he didn't, he had books for a reason. But he went there anyway, mostly to make mommy happy. Additionally, he chose to get a degree in Physics, to annoy Mycroft, who couldn't stand their parents being so happy with their little son, all grown up and responsible and getting a real degree, unlike Mycroft's, who chose Art History. Although most of his classes were boring at best, the equipment his school provided was enough to keep Sherlock happy and not insulting his classmates more than acceptable.
When Sherlock was twenty-five, he was out of college and out of his mind. The dullness of his previous life was nothing compared to the experience he got after he finished his education. Every job he tried was horribly boring, people were more than irritating and soon Sherlock couldn't stand to see anybody besides his family, but even his relatives' visits were severely restricted. Sherlock could go for days without speaking to anybody, he hardly ate and hardly slept, spending his time reading books and looking for other publications, the ones he hadn't read yet. But it couldn't last forever, so Sherlock went to look for his own thrill, a thrill to take away the dull ache of life. Life without job, without adventure, without stimulus. It didn't take him long to experience drugs and alcohol. The intoxication had him feeling extremely odd, yet he quickly got hooked. He expected himself to hate the lightheadedness and unclear mind that drugs and alcohol induced, but he didn't. Finally, there was something to stop this train of thoughts that was beeping constantly in his head. And even though Sherlock realized that it wasn't a gentle stop, but more a concrete wall the train has crashed into, he didn't care. His life was dull and boring, but at least he wasn't conscious enough to really feel it.
When Sherlock was 30, he was already out of rehab and on his two feet. He was working for Lestrade, who had pretty much saved him, sleeping and eating occasionally and trying his best not to overdose again. His reasons were trivial- he didn't want to see the disapproving look on his brother's face once again. Sherlock really tried to make the job for Lestrade everything that he needed, but it wasn't enough. He tried to dig up cold cases and solve them, tried out many experiments, tried to do anything, anything, to stop being bored but nothing was enough. But Sherlock pushed through, not entirely sure what for.
But now Sherlock is thirty-five and he knows exactly what for. It's been some time already that he found a new flat mate, John Watson, who then proceeded to accompany him on cases and write a blog about the two of them. John buys milk, arranges meetings with Mrs Hudson, cooks dinner sometimes or simply reminds Sherlock to eat, makes great tea, handles Mycroft and Anderson and is there every time Sherlock needs a text to be sent. But most importantly, John is a friend. The missing link between Sherlock and the world, between his brilliant mind and harsh reality.