It's not a question if it happens, because it will. That much has been obvious since the beginning. It's not a question of how it happens; because when it does that will be an insignificant detail. And it's not a question of when it happens, because you and I both know it will happen soon enough. The question, then, is why it happens, why it has been written in the stars, in the unspoken avowals, in the half-realised glances, in the allure, since that bitter January morning in a dim hospital laboratory.

The question, here, is why they are going to kiss.

Why, you must be wondering, is it going to happen?

Because John shot a man for Sherlock Holmes the very day he met him, the very day, and isn't that mad, isn't that the most insane thing you could possibly conceive of; shooting someone for someone else you've only just met, and it seems ludicrous, but not for John and Sherlock. You look at them and think, My god, they would find each other in every age in history, they would die for each other, and you're right. In the end [pool, roof, bomb, assassin], they do.

Because Sherlock wants to kiss John every time he looks at him. They are running and it's dark and breathlessly cold, and he wants to kiss him. They are sitting in the living room with flames dancing in the grate, and he wants to kiss him. They are scrutinizing a corpse and it's raining fucking buckets, and he wants to kiss him. They are walking to Angelo's with their shoulders bumping companionably together, and he wants to kiss him. They are fighting about the pancreas in the cupboard, and he wants to kiss him. A palm sliding along his knee and "I don't mind," and he wants to kiss him.

Because, my dear, John looks at Sherlock and he doesn't see the addict or the sociopath or the failure, but scalding brilliance made of perambulating galaxies and dust from the Big Bang and flaming comets leftover from Galileo's time. It would be laughably dramatic if it weren't so exact, so accurate a rendering of Sherlock, of Sherlock-in-John-Watson's-eyes.

Because, darling, there's something between them. The Something is both radiant and painful, wrecked and lovely, and though they try hard as hell to ignore it, it's only getting inevitably stronger. A silken, wild energy runs from Sherlock to John to Sherlock to John to Sherlock to John, endless, zinging to and fro, back and forth, every day they are alive. When Sherlock was dead, John's seemed to burst out of a place in him as dark as the wolf's mouth and spin into nothingness, its destination buried six feet under, knowing nothing of the faked fall or the resurrection to come. Just blackness and a severed cord. The Something goes into hibernation during those ungodly three years, but it too reappears in the light, beautiful and strangely difficult to look at.

Why do they kiss?

Sherlock will say it's because people are slaves to the whims of hormones and chemistry, biology and anatomy and science. He will not talk about the way John has wrapped his heart, no. He will look down the lens of his microscope like a marble angel and you will see nothing of that. Just as he wants it.

But you will know better.

John will say it's because they are bonded at such a level they cannot survive without the other, that they are the end and beginning of all there is: heat, danger, bloody heartbeats, hands grasping so tight they bruise, eyes meeting unerringly, shoulders shaking with grief and laughter and a gory mix of the two, starlight on rough seas, crescendo, eruption, craving, cure, intimacy from which they'll never recover.

His explanation will be far closer to the truth.

Maybe it happens when Sherlock staggers off the airplane back onto Heathrow tarmac, dizzy and cracked open, squinting into the light of the sun, of the second chance. He will approach the Watson's, bypass A.G.R.A, and pull John in by the lapels. Maybe it happens much later. They'll be sitting in 221b just the two of them, sipping whiskey and polishing John's old handgun, flushed and laughing with their feet entangled beneath their chairs, palpably infatuated—though neither of them will say it—and John will get a bit sloppy and a bit too close to Sherlock's slumped form (only Sherlock won't notice that, just the way his pulse quickens and his core thaws to magma) and John will say outright, in a voice thick with potential, with want, "Can I kiss you," and Sherlock will be wordless and they will put their mouths on each other across the kitchen table, mouths and teeth and tongue and saliva, panting, purloining each other's carbon dioxide, and neither will be thinking of Mary.

Perhaps, lovely, the important thing is not why they kiss after all, but simply that they do, stubble and need and "Oh God, just there, just th-there."

In the fathomless, aortal dance of our universe, maybe that is the only act that matters.