Sometimes John worried about his flatmate. He worried when he found body parts in the fridge and the toaster, all put there in the name of science.
He worries when he found empty boxes of nicotine patches after a particularly strenuous case and always felt that between waking Sherlock up to scold him and force him to eat and letting him sleep, there really was no right answer.
He worries when Mycroft mentioned in passing that despite being adopted at a young age, Sherlock had never really bonded with anyone other than their mother, though whether that was made better or worse by his stay in an asylum when he was a in an adolescent was questionable. After all, it had seemed for a time that he had it in him to be cruel rather than just detached.
He worries when Lestrade refused to answer questions about Sherlock's past drug abuse.
He worries when Sherlock looked at dead and mutilated bodies, and even those still living and suffering, with flat, uncomprehending eyes. He worries when those same eyes lit up at the mention of murder already casing down the threads of the mystery behind it.
He worried when the police under Lestrade mock Sherlock, and wonder if any of them knows how much of himself he pours into a case.
He worries when his flatmate stares into space for hours at a time or plays a violin until he draws blood and longer until John stops him.
He worries when Sherlock looks at him with a rare open expression and seems to crave human touch. He worries more when every time John goes to touch him he flinches away.
He spends so much time worrying about his flatmate that he eventually arrives at the conclusion that he should stop. Despite all of his peculiarities, Sherlock has arrived at a his present age more or less physically intact and mentally intact. Well, perhaps less more than not. But still, despite being both a doctor and the closest thing to a friend Sherlock Holmes has, it was not his responsibility to let worry for his flatmate define every hour of his life. He could try to help him in every way he could, but this never-ending, gut-twisting worry did neither of them any good.
It was about the time that John arrived at this conclusion, that he noticed an intermittent and not so very peculiar habit of Sherlock's. Sometimes, and not at any particular times, but sporadically, once while on a case sometimes while lying about the flat, Sherlock would being tapping. A rapid, dah-dah-da-da. For such an absent minded habit, it was always very sharp and precise. When John questioned Sherlock about the tapping, he looked at the offending digit, still rapping away on the edge of the sofa and said half dreamily, "Oh, it's just the sound of drums."
For some reason, after that, John felt much more inclined to spend his time worrying about his flatmate, just so long as he didn't have to acknowledge that echoing tap of drums.