This is my first foray into the Sherlock fandom. I have so many problems

He obsessed over it. It was only natural, really, Sherlock's mind like an info-processing system: lightening-fast, inexorable, perpetual; the whole thing turning inward with destruction in the margins if there was not some stimulating new query to be answered. And he had just such a problem now.

It wasn't the how, that answer would come soon enough, through the right channels in due time. It wasn't the what, since that had been spelled out quite clearly enough, and he didn't need it repeated. The where was immaterial and the who had already been solved and the when was half the fun. It was the why.

The why that gripped his senses, the why that ran dizzying circles behind his eyelids, ground his mind like gears with no oil, the smoke and burn of endless friction with no resolve. Because it was actually the why to the wrong question. He should be focused on Moriarty. All faculties and brain power dedicated to this war he'd unintentionally started. But instead all he can think about is a flak vest strapped with Semtex, an overcoat two sizes too big, the clean, harsh scent of chlorine overlaying fear, and a glittering red dot over a heart that wasn't his because, to be fair, he didn't have one. Until now, apparently.

Sherlock understood emotions, in a theoretical capacity. He understood attraction, happiness, anger, sorrow, empathy; understood, traditionally, which situations called for which reactions, and he'd studied it all so well he could even mimic them on cue. He understands that humans feel. He can even admit sometimes, when no one else is in the room, that he occasionally feels, too. He just doesn't know why.

And that's the rub.

"However much it probably pains you," John says one night, a rough but soothing thumb pressing lightly into the sagging shades of purple under one bloodshot eye, and his tone might even be wry if his face didn't look so tragic, "but some things just don't have a reason why."

Ah, John. He's wrong, of course. There is a reason for everything, even if mere mortals are incapable of grasping it. Coincidence doesn't exist. Chance is for the superstitious fools. Every effect has a cause because nothing happens in an independent vacuum. Nothing is as random as it seems. But because he's John, though he's missed the point entirely, he's still essentially right.

Beyond the biological imperatives and evolutionary byproducts that create easily quantifiable instincts, nothing adequately explains the raw, gnashing cold of knowing for a certainty the one you care about shall die. Nothing explains that. Nothing comes close, and all the anthropological evidence in the world just feels like a house of cards; some paper-thin veneer to hide behind and ignore what's really happening. The real reason why. It's a matroyshka doll of why's and why's and why's, each reason giving way to more questions until it all loops back on itself because there is nothing concrete to ground it. Nothing measurable, nothing provable.

No data.

There are plenty of things Sherlock doesn't understand, for all the many facts he knows. He doesn't understand why a sometime-acquaintance thought an ex-Army doctor would make a satisfactory flatmate. He doesn't know why Lestrade never checks in the spare tea kettle when he knows very well what is in it. He doesn't know why Molly still sought his attention, or why Mycroft doesn't simply bend Sherlock to his will like he did all others who stood in his way, or why Mrs. Hudson hasn't evicted him yet. He doesn't know why John is still here. Doesn't know why he remains, through mockery and disappointment and insults and petty fights about nothing that somehow morph into everything; doesn't know why the man killed for him after only just meeting him, why he follows his every command, or why after enduring all of that, he would still throw his life away without a second thought to save Sherlock's, or why the mere memory of it makes him sick.

But he does know why Moriarty is a genius in his own right. Because he knew, before Sherlock even had it figured out, that Sherlock didn't have a heart. Not of his own, anyway, as he'd apparently been hiding the thing in John Watson for safe keeping, so safe even Sherlock didn't know it, and if he didn't know, how could anyone else? But he'd underestimated his quarry. Badly. And now Moriarty knew that the most efficient way to burn Sherlock Holmes' heart was to stop John's.

And that, most probably, was the why.