Sherlock looks at John's slumped shoulders, the flat, pinched line of his mouth, and something cold and horrible curls up in his gut.
"I've disappointed you," he says, as neutrally as he knows how.
John laughs mirthlessly, doesn't bother denying what they both know. "That's good, that's a good deduction, that," he says bitterly.
John, disappointed in him. His blood, never his best friend, condenses to frozen sludge in his veins. "Don't make people into heroes, John, heroes don't exist; and if they did, I wouldn't be one of them," he snaps, levelly, striking back before John can say anything more, can drive it home just how little Sherlock could possibly hope to gain John's forgiveness, his affections. It was stupid, anyway, stupid to think of it. He should have seen it coming; should have snipped it in the bud. He could see that hero-worship grow in John, case after case, day after day, with every "That was amazing" that John let slip carelessly, uncaring what it meant, for himself.
He can't actually look at John after that, has to create a distraction, immediately, regain some worth. He focuses on the Pink Lady's phone, takes it up and starts typing, niggling at the clue until it gives up and reveals itself for him, gives him the means to see that look in John's eyes again.
He should have known better, an old addict like himself. He knows the signs; how has he let John slip so far under his skin that Sherlock craves him like he craves another hit, dreads John's disappointment like a month without a new case, has to push himself further and further just to have John look at him like that again, like he's worth something, like he's incredible, like he isn't just a pathetic excuse for a human being.
Which he is; he knows it. Everyone he's ever met can't be wrong, and John is just an anomaly, a wonderful, precious anomaly that Sherlock wants to protect, cultivate, never let down. But he can't. He isn't capable of that, of keeping something like that, nurturing it and having it for his own. How, how could John possibly have it so wrong; the dichotomy eats at him, aches somewhere behind his breastbone. How could John think Sherlock is the hero in this partnership? How could he not know what he is, how could he think so little of himself as to compare what he is to Sherlock? Sherlock wasn't lying when he told John that he would never be one of the heroes, but he was lying through his teeth when he said that heroes didn't exist, so plainly that he doesn't know how something didn't strike him down for it. Because it's blindingly obvious that heroes do exist -- he sees the proof every day; it lives with him, for goodness' sake, works with him, protects him, makes the world worth saving.
The fact that John is so blind to the truth makes Sherlock sick inside. Because he knows, like he knows that the charcoal traces on the fingers of the left hand of the man who lives across the street from them mean that he's a left-handed artist who has just sold his first artwork to a major gallery, that he is going to let John down. That sooner or later, John will find out that apart from his brilliant brain, Sherlock is nothing, worthless; that without cases to solve and secrets to puzzle out, there's nothing to him.
And yes, Sherlock knows that John will leave him eventually, when he does figure it out; but perhaps Sherlock could postpone the moment by taking the pains to stop John's delusions, that Sherlock is anything approximating a hero, anything as good and amazing and incredible and worthy of admiration as John is himself. If Sherlock isn't John's hero, he can never let him down.
Sherlock can live with himself quite well enough; he doesn't bother lying to himself about what he is, and just how unreachable that makes John. Because John, he deserves so much better than a broken man holding the world's second-finest brain (he isn't even the best; no, Mycroft is, as always, just that one step further up from him, and god, does that sting). There's nothing he can offer John that would be worthy, and he knows it. He can live with that. As long as he has even a fraction of John's attention, John's presence in his life, it'll be enough.
With that in mind, he sets his one asset firmly on finding Moriarty, and disposing of him before Moriarty can succeed in his task and show John just what Sherlock really is. Because, to Sherlock's surprise, there is one thing in this world he would mind losing, and mind strongly enough to make sure it never happens -- even if it means depriving himself of John's admiration, that misguided hero-worship that has brightened his life ever since John walked into it. Besides, Sherlock himself holds more than enough hero-worship to balance it out.