It's just fingers, the first time.
John's fingers on his arm. Not quite pulling, not quite restraining, but insistent and warm and real, the tips pressing in just slightly.
Except it's not just fingers, because it's eyes, too.
John's eyes. Looking at him like–
Sherlock's chest constricts and his head swims with a combination of panic and something else entirely.
And it's in that instant that Sherlock knows he's been wrong.
Wrong, wrong, wrong.
Because once up on a time, that look was everything he swore he'd never want.
Now it's just everything.
And instinctively, Sherlock knows that that's how he wants to be looking at John. Like the doctor is the center of his universe, the sole thing deserving his attention. Like John's fate will be Sherlock's fate, always. Like he lov–
But he can't. Doesn't know how. It's all there, swelling in his chest and unfurling like warmth where John's hand rests on his arm, but he just can't– he doesn't kno–
"Sherlock."
And then it's John's voice, low and sure, drawing the genius out of his panic and back to the darkened warehouse. Back to the stack of boxes they're hiding behind and the criminals they've been chasing.
"Sherlock, are you certain?"
And if John is aware that Sherlock's breathy "Yes." answers more than one question, he doesn't show it. Instead, he lets his hand drop in the interest of drawing his gun and moves, swift and silent, after their quarry.
Later, when the chase is over and the case is solved, Sherlock imagines that he can still feel the heat of John's grip through his sleeve, and wonders what it might be like if John were to hold on and never let go.