An adventure across the universe, that's what he promised. That's what the Doctor promised to an 8 year old Sherlock Holmes. But the promise wasn't kept. 5 minutes he said that he would be back. He was sure of it. The Doctor felt his minutes, but Sherlock felt the years. Years he waited for the mysterious man to appear with his disappearing blue box, to take him to an adventure across the universe. But with each day, of every year, he was let down.
Sherlock was special He saw things differently than what people did. He was smarter, and he everyone with detail… and he can forget things easier than anyone by just deleting them from his memory. And that's what he did. With every passing day, he lost more and more hope of being taken on an adventure. He let go of that dream, and deleted the Doctor. But he didn't stop there; he deleted the Doctor along with any memory and information. that contained the stars and planets. Just because of the 5 minutes the Doctor had promised. What Sherlock didn't know, is that memories can reappear by just triggering something. A smell, sound… or word.
Sherlock has heard of the mysterious things happening in England. Of a man with a blue box, but no one has mentioned a name. Sherlock was attracted to this mysterious blue box, he couldn't find the answer. This man would pop in and out of historical records. Sherlock couldn't believe it, until a sudden run in with a killer cabbie.
"There's others out there just like you except you're just a man, and they're so much more…" Sherlock pondered with the words said to him, "what do you mean, more than a man." The cabbie smiled at him, "there's a name, no one says, but I' m not going to say it either." "Give me a name!"
The cabbie knew something, but he wouldn't say. Nor would he ever say it, for once John Watson shot the man, he died with the information Sherlock wanted to know so desperately. What others? There's people out there just like him, but they were more than a man. He thought of all this, and he looked at all the news paper clippings he gathered that were tacked onto his wall. All the internet sites and photo's of the same blue police call box… when a wheezing noise interrupted his thoughts.
Most people would remember their forgetting memories but just looking at something. Sherlock Holmes was a different case. He managed to delete the information stored in his brain, but you can't really destroy something that never really existed. Sherlock needed more than a sighting of a blue box. There are millions of the blue boxes in the world, but there was only one Doctor. And that was his memory trigger, the Doctor.
When he saw the box appear in front of 221B, he went to investigate. The Doctor found out about Sherlock Holmes. He remembered the boy with the curly hair and bright curious eyes. The boy he kept waiting for what he thought was to be 5 minutes, when in reality was 29 years. But here's the thing about the time-lord. He always keeps his promises. He couldn't return in 5 minutes, but he did promise and adventure and he was going to keep that promise. He appeared in front of Sherlock's flat. He was well aware that Sherlock might of forgotten the memory from the question he asked once he saw the Doctor, "who are you?"
Knowing how curious Sherlock Holmes was, the Doctor disappeared back inside the TARDIS. He knew Sherlock would walk inside. Which he did. Sherlock didn't know what he was in. He didn't know that this was a huge role in which why he started investigating. He always though, and believed, it was because the police always got the facts wrong and were always too slow. Sherlock solved his first crime when he was 8, because he was looking for the blue box, but found a crime scene and quickly solved it. If he wasn't TARDIS hunting, he wouldn't be the consulting detective he was today.
After minutes of talking back and forth, Sherlock finally asked the question once more and got the answer he needed, "who are you?" The Doctor smiled brightly at Sherlock, "I'm the Doctor." Once those words left his mouth, something in Sherlock's head clicked. The 5 minutes, the blue box, the wheezing noise, everything about the stars and planets and the promise rushed through his mind. The Doctor smiled, and started pressing buttons and flipping switches on the control panel.
The Doctor took Sherlock on the adventure of his lifetime. Across the universe and anywhere in time. So that answers your question on why Sherlock Holmes erased the universe from his mind. Now, people ask how Sherlock survived the fall… The Doctor might have had a part in that as well. But, that's another story for a different time…