Sherlock would never be one to deny he thought a lot about things.
He just generally avoided advertising he thought a lot about John.
Sherlock didn't think John was much aware of it - admittedly, most of the time his whirlwind of a mind was occupied with a case or some puzzle or whatever experiment was particularly critical at the moment. But at some point, his eyes would alight on the man that a part of him couldn't help but call friend... and there he could do but pause to contemplate the most baffling mystery of all.
Sometimes, he recalled their first few months living together - honeymoon phase. John fascinated with Sherlock. Sherlock fascinated with John. Eventually, the novelty had dimmed of course - novelty always did - yet they remained bound together by their love of the chase and perhaps some other inexplicable thing - so inexplicable, Sherlock often felt that it must be a mirage. That if he were ever to extend his hand to its brilliance and grasp it, everything would shatter into sand. So he never found the courage to try.
After all, between not noticing when his flatmate left and having an altogether cynical view on just about everything, Sherlock tended to wonder why John stayed. Nowadays, there were no murmurs of amazement each time they went out to solve a crime, building to the probability that John had finally joined the ranks of the general population in finding the detective far too irritating to be intriguing. Sherlock had expected it - expected John to one day become normal. And yet, there was that strange, worrying fact that somehow... John had never stopped fascinating Sherlock. For some reason, he wasn't always able to predict John's opinions, his reactions, his decisions. Every single time Sherlock thought he had him thoroughly figured out, John came up with some new facet of himself that made half of his theories crumble to the ground.
It was usually in his eyes, Sherlock thought. Bone-deep blue that in the darkest moments seemed to involuntarily shine back at him with something... something Sherlock didn't understand. Something Sherlock didn't think he'd ever understand. Something that especially appeared within them whenever John risked his life to defend him - save him - despite anything and everything that would, in any other relationship, break two people irrevocably apart. Don't you see? they always said. But then it was as if they carried out their explanation in another language entirely because no, Sherlock never saw.
Except for that last time, when he found himself brandishing a handgun at a man whose neck John had captured in his arm. He didn't quite hear the command to run. His mind had chosen that instant to be preoccupied with some curious revelation - that perhaps he'd never recognized that look because the only other place it had claimed was the space behind his own eyes; that he hadn't quite realised just how much John had given him because of it.
And when, not too long after, he steadily pointed his weapon at the bomb on the ground, he couldn't help but wonder if he might ever earn the chance to give it back.