Everything changes at one point.
He can't tell if it was when Sherlock shoved a hat from some poor actor's dressing room into his face, or maybe when he came home on the Tube with a bloody harpoon, or maybe when Sherlock swanned off a building only to return three years later with new scars and a haunted face.
Or maybe when he killed a murderous cabbie that very first adventure.
Either way, everything had changed.
And now, lying in the arms of Greg Lestrade, bleeding out onto the rubbish and piss, he recalls a conversation he had with Sherlock –
"Oh use your imagination."
"I don't have to."
His last words have been planned in his head for ages at this point. No need for imagination when he's been in this situation plenty of times over the years. But it looks like he'll actually get to use them this time.
"Greg…Greg…Don't let me die until I've told Sherlock I love him."
"John, come on now, the ambulance–"
He's shaking now.
"Don't. Don't let me die until I've told Sherlock I love him."
He doesn't.