If the Sherlock we imagine existed—the sweet one who is tender to John—I think a fight like this would eventually have to happen, what with John seeing how kind Sherlock can be, yet witnessing daily how awful he often chooses to be instead. Lestrade said Sherlock was a great man, someday he might even be a good one; while Mycroft thinks John could be the making of his brother. I think John will be Sherlock's catalyst for change, change for the better, but it's going to hurt along the way. This is how I'm imagining it starts. Don't worry, it ends well. Spectacularly well if you must know.
Chapter 1: John Watson Has a Mad
It was the searing black and white of it that started everything.
The white: A morning that had started slowly, in bed, and with kisses.
Sherlock was always sweetly hungry for any affection John gave him, as delighted as a child with a rare gift. So when John announced that morning that no one was allowed out of bed until he'd kissed all of Sherlock's "bumpy bits," the tall, lean man had stretched like a cat, then done a very sexy imitation of a purr.
With a smile John kept his word, kissing Sherlock's ankles, both knees, his hip bones, his elbows, collar bones and ribs, and finally his Adam's apple and nose.
"And now, if you turn over I'll take care of that spine and your shoulder blades. And quite possibly other things."
But Sherlock didn't turn over, instead he pulled John to him, wrapped him up in long arms, and held him tightly for the next twenty minutes.
The black came two hours later, undoing everything that had come before.
"—and you really shouldn't open that over-large mouth—by the way, your dentist misses you, and you may want to do something about your breath, it could peel the skin from a skull—if you're so clearly unable to think before you speak madam."
John stood rooted in the doorway of the kitchen, jaw dropped, tea mugs in both hands.
In the living room, not quite a dozen feet away, sat his sister Harry and Sherlock. They had met for the first time not quite two minutes ago and had been alone for exactly 23 seconds. It was the latter who had just now eviscerated the former.
John said nothing to either of them, just turned on his heels and left.
No one saw him for the next ten hours. By then Harry had gone home, Sherlock was setting up an experiment, and John was frozen half to death, having walked a large part of central London in a coat more suitable for taxi rides.
It was long after nightfall when he returned to the flat, only to discover the living room mercifully empty. With a relieved sigh he hung his head a moment and rubbed his eyes. Tension he'd been carrying most of the day started draining from his body.
His very cold body.
He crossed the living room quickly, toward the mostly-working radiator left of the moose head. He groaned with relief at the warmth, chafing his hands and rubbing his arms. It was then he noticed Sherlock's reflection in the window glass.
Sitting at the kitchen table, a small desk lamp blaring across the tabletop and his busy hands, his lover said, "You should take those clothes off. Your feet will be ice later." The detective smiled to himself. "You could take them off there if you like. I wouldn't mind."
For a long time John just stared at the other man's reflection, saying nothing as the tension flooded back into his body, making his posture rigid, and his hand shake. Finally he spoke, measured and low. "You do what you're good at."
Sherlock plucked up a pair of tweezers and said, "Thank you."
John's gaze fell to the street. It was raining now. Icy rain that shifted into sleet and back again. It was the kind of weather that probably kept even criminals indoors. "That wasn't a compliment."
Sherlock didn't hear the reply at first, intent on tweezing spider eggs into a tub of clotted cream. After a moment his brows tugged down, and with a blink he looked up. "Why are you mad at me?"
When you were spoiling for a fight it did save time, living with Sherlock Holmes. He always skipped the banal lead ups—"Excuse me, what did you say?" or "What is that supposed to mean?"—and went right to the meat of the matter.
John watched a couple down below run across the slick road. The woman caught the man's hand when he slipped. "You spend so much time telling the rest of us we're stupid. But it's really you who's stupid."
Sherlock pursed his lips, glanced at his spider eggs, back to John, then put down the tweezers and waited.
"You don't know about the solar system, that one's well-known." John gently kicked at the wall, unaware he was doing it. "You're also completely ignorant of everyday etiquette, we all know that's another, and you—"
Sherlock clenched his jaw and frowned at his flatmate's back. "Is there a point to this John?"
The doctor turned from the window, took a deep breath. "Okay, here it is: You do what's easy for you Sherlock. You do what's easy. For you magnificent deduction is almost as simple breathing. You're not even trying hard most of the time." John's hands fisted, he was warming up to this. "But then you look at everyone else, people who are trying, trying very hard to be better than they are and you belittle them. Where do you get off minimizing everyone else when You. Aren't. Even. Trying. When was the last time you actually did something that's hard for you, something that doesn't come naturally? Ever? Ever?"
When the words weren't laudatory or in some other way interesting, Sherlock often tuned people out. He did this quickly, protectively, and by now instinctively. To nearly everyone. All the time.
And frankly it was usually for the best, because it was rare moments like these when the consulting detective understood how much power words actually have. You can't touch them, you can't taste them, but they can leave you with a sharp ache in your belly and bitterness in your mouth just the same.
Sherlock blinked rapidly. "I don't understand," he said.
John barked out a laugh. "Ha! Well then clearly you're an idiot. Certainly stupider than anyone else in the room, that's for sure."
Sherlock opened his mouth. His heart was pounding so hard it was difficult to breathe.
"Does it feel good, Sherlock, feeling stupid? That coldness in the pit of your stomach that actually hurts, your throat tight and closed off, as if you can't swallow?"
Sherlock squeezed his eyes closed.
"You. Don't. Try. And you mock those of us who do." Complete silence for a thousand years. Maybe two. And then: "And I am so sick of it."
There was screaming in his head now, such terrible screaming. Here it is here it is here it is oh god it's happening. John is leaving.
"Use your words."
Sherlock jumped, eyes flying open. John was right there, in his face, leaning across the kitchen table and sneering. Sneering. Sherlock never realized how ugly it could look.
"Go ahead Sherlock, use your words. God knows you have so many, soooooo very many. Use some now, say something."
Sherlock groaned softly, certain something was tearing inside, nothing could hurt this sharply, this suddenly, unless there was real, physical—
"Tell me I'm wrong. Tell me off. Do what you know. Do what comes so easily for you. Go ahead Sherlock, just do the awful things you're so good at."
Finally there was a sound from the other man, a low, low static-y sound, the sharp, small, terrible sound of a heart breaking.