He'd done what Sherlock had asked, despite every ounce of his being fighting against it. Fighting the urge to run inside and pull him off the ledge, knowing that he wouldn't be fast enough. Fighting the urge to sprout wings and fly up there to carry him down to safety.
But then Sherlock had asked, and there was nothing else he could do.
"Keep your eyes fixed on me. Please, will you do this for me?"
He'd done that for Sherlock.
But now there was one more thing he wanted. Because he'd listened, he'd kept his eyes fixed on Sherlock, like a car crash you couldn't stop watching.
And he'd watched Sherlock fall.
He'd done that for him.
So in return, Sherlock could do one thing for John. Just one more thing.
So he told him.
"There's just one more thing, mate, one more thing. One more miracle, Sherlock, for me. Don't be dead. Would you? Just for me, just stop it. Stop this."
Because it would only be fair.
Quid pro quo.