Based on “The Sontaran Stratagem” dialogue.
After the fall, Sherlock had a very peculiar visitor at his new lodgings. He came quite by surprise and, without waiting for an invitation, walked right into the small flat that Sherlock now lived in and settled himself into Sherlock’s own chair, surveying him closely.
“Sherlock Holmes…” he mused, rubbing his chin. “Good God, you’ve gotten tall.”
Sherlock froze, his hand immediately reaching for the gun at his hip. He rarely ever went anywhere without it now, what with the surprise attacks (if you could call them surprises) he had been greeted with in the past.
“Ah, no need for that,” the man said, waving a hand airily. “Never liked guns. Too loud.”
“Who are you.”
The man’s forehead creased slightly, eyebrows drawing together in a frown. “You don’t remember me?”
“If I did, I wouldn’t be asking.”
“I suppose it was quite a while ago. Years by the look of it,” the man smiled. “I’m the Doctor.”
“Am I supposed to be impressed?” Sherlock asked after a moment, quirking an eyebrow.
“You should be, yes,” the man replied, examining his fingernails. “But we can get to that later. From what I heard, you’re dead.”
Sherlock looked down at his body, shaking an arm deliberately. “As far as I can tell, I’m alive.”
The Doctor laughed before growing serious. “Yes, you are. Amazing, isn’t it?”
Sherlock scoffed. “What’s so amazing about it?”
“What, you don’t think living’s amazing?”
“Not particularly, no. It’s so hatefully boring.”
The Doctor got up quickly, his frown deepening. “You don’t mean that.”
Sherlock crossed his arms, taking a step forward. “And how do you know what I mean? How I think?”
“Because, like you, I’m clever. In fact, I’m probably much more clever than you,” the man said. Sherlock opened his mouth to retort, but the Doctor held a hand up to silence him. “Not so easy is it…being clever,” he said softly, moving forward. “You look at the world and you connect things…random things…and think ‘why can’t anyone else see it?’ The rest of the world is so…slow.”
Sherlock froze. This was almost exactly the same conversation he had had with the cabbie years ago. Why can’t people just think?
“Yes,” he whispered. Why couldn’t people just think? Was it that hard?
The Doctor took another step forward. “And you’re all on your own,” he said, his voice carrying the sorrow of a man who had seen too much of the destructions life brought. “It isn’t easy at all, is it?”
Sherlock swallowed, looking away.
“Alone is what I have,” he said after a moment, knowing with every word he said that it was completely untrue. “Alone protects me.”
At this, the Doctor’s eyes seemed to well up with emotion, the sadness of centuries of loneliness surfacing for one single, solitary moment before disappearing.
“You and I both know that that’s not quite true.”
Sherlock looked back up at the Doctor. “Then tell me, Doctor. Why are you alone?”
The Doctor was quiet for a moment, a small, sad smile playing on his face. “Because it’s too late for me,” he said softly before looking back up, eyes blazing. “But it’s not too late for you, Sherlock Holmes. It’s never too late for you.”