Sherlock is trying to apologize.  And doing a terrible job.  John is entirely out of patience.  He starts to walk away.  

He wonders if Sherlock can perceive the magnitude of his anger from his physiological responses.  Increased blood flow?  Tension in his jaw and stance?  John hopes so, because he doesn't feel much like putting it into words.

Perhaps he can, because Sherlock calls after him.  "Listen, what I said before, John.... I meant it."

John turns to stare at him in disbelief.  What he'd said before was more than a bit not good.

"I don't have friends."  Sherlock repeats.  He bites his lip.  "I've just got one."

John thinks about it, nods.  Feels his tension ease, just a bit.  "Right."  Then he keeps walking.  It's a start.  But he wants to explain to Sherlock—preferably while grabbing him and shaking him—all the problems with his previous statement.

One.  You can't just take friends for granted, expect that they'll still be your friend even after you do something terrible.  Maybe you can a little bit, sometimes—after leaving heads in the fridge, or inviting yourself along on dates—but not for the really big things.  Not after pushing your friend away, disavowing their importance.

Two.  You especially shouldn't do that when you could really use a friend.  When you're afraid.  When you're not able to trust yourself.  That's exactly when you should be turning to the person you're closest to for help, not chasing them away.

Three.  You especially, especially shouldn't do that when you only have one friend.  Really, even Sherlock should be able to figure that one out.

And, finally, four.  John takes a deep breath.  "Friend" doesn't even begin to describe their relationship anymore.  Maybe Sherlock doesn't know, not having any other friends, but John has done some reflecting, since Irene.  He has been forced, eventually, to agree with her assessment that they are, in fact, more like a couple than just friends; he didn't even bother to deny it to the owners of the bed and breakfast, except for a fitful, habitual beginning of a protest.  He has built a life with Sherlock — built his whole life around Sherlock, really, though Sherlock has changed to accommodate John, as well.  He will happily keep building that life together indefinitely—and would even share a double room, if Sherlock were amenable.  Would definitely share, actually.  He's done quite a bit of thinking about such things, since Irene.

But John can only build this life if Sherlock will take more care with his heart.  He can't take being jerked around, not to this extent.  Being put in danger, yes.  But having their relationship dismissed, denied, entirely?  You don't do that to friends.  Or to... partners.

He wants to tell Sherlock that.  All of it.  But he doesn't know how.  It's too big a conversation to have.  Perhaps he should have begun some of it after Irene, but he needed to take the time to think it all through, slowly and carefully, as is his wont.  And he's too scared of where it will all lead.  If Sherlock feels similarly, but especially if he doesn't.  There are things he wants to hear from Sherlock, if he opens himself up and admits everything, but he isn't sure if Sherlock is capable of those responses.

Maybe someday he'll broach the topic.  For now, he settles for eliciting further apologies and compliments from Sherlock, knowing how rare the opportunity is.

"John? John!"  Sherlock chases after him as he walks away.  "You are amazing!  You are fantastic!"  John smiles to himself.  It's not everything he wants to hear Sherlock say, but it's not bad.  

He tells Sherlock not to overdo it, but he carries on:  "You've never been the most luminous of people, but as a conductor of light you are unbeatable."  John sighs.  Someday, he'll have to work with Sherlock on his apologies.