Prompt fill from sherlockbbc-fic which reads: Sherlock usually gets bored of people after a while - his relationships end when he stops finding interesting things about them, or when the need for novelty overwhelms all else. Ten years or so down the line, he's still together with John, and he's bored a lot. Show me a moment where Sherlock remembers why he's still with John, why he loves him, why he's spent so much of his life with the same man and why he's going to spend the rest of it with him, too.

From the sofa, Sherlock watches John through narrowed eyes. He is in one of his... moods.

(Why now, why today?)

He hates these thoughts that enter his brain unbidden that make him doubt the life he has created with John, the life they have made together.

He's been given this gift, this epic love, that is John Watson and he is an awful person to not appreciate it.

At first it was so exciting, to untangle all the secret threads that John had within him, applying his methods to deduce everything about this man who was interesting and full of surprises and definitely not-boring.

But what to do now that he had learned all he could?

Sherlock is not good at relationships. For him, there is no guiding precedent. He doesn't know what comes next: should they press on without the novelty? Or is this a sign that the relationship has run its natural course and should be terminated? This isn't about physical passion - that's still there, he still thrills at John's slightest touch - but he knows himself and he's been anticipating the mental boredom.

More questions come: Why do couples stay together? Is it the history they have shared, knowing each other's secrets, the acceptance of faults which others would reject? Or is it simple attachment that binds us to another person?

Sherlock knows that the concept of attachment merely divides down to a smattering of chemistry, namely oxytocin and vasopressin. That is the logical answer, the one that can be proven with empirical data.

Once, he would have been satisfied with that answer. He would have reached the logical conclusion and just stopped there. But now, with years of John's humanizing influence, he knows to go beyond the empirical and uncover the arcane.

Arcane... Esoteric. Cabalistic. These are words that Sherlock usually shies away from, along with instinct and intuition and indefinite. He's not comfortable with these wisps of words that vanish like mist when you try to grasp them. His language is cold and precise: one of reason, mathematics, formulas, logic, data, experiments, and scientific methods. However, he concedes, perhaps not everything needs to be picked apart and analyzed. Look at John. He is content just to be.

But maybe not. Maybe that's only an illusion. Sherlock wonders: Does John ever have doubts?

Impossible. John is a good man with a pure shining heart and he would never have an unkind thought.

Sherlock drifts back to his life before John and it was like he had been asleep for all those years, blissfully unaware of what he was lacking. Then John came along and showed him the power of caring about another person. And it's not as if he is interested in finding a replacement, a new mystery. No, it was John or it was nobody.

He understands that he could never choose a life without John, not after knowing what riches he had possessed and pushed away. That path would lead to loneliness, isolation, melancholy, drugs. He would revert to the way he was before, possibly retreat farther than ever, and all the good that John has brought about would be undone.

(Without you I am less than one but together we make more than two.)

It's the strangest mathematical equation he has ever encountered. It can't be proven on paper, no such formula exists, but he knows with all his brain and heart that it is true.

(Show me how it's true. Prove it. I must have data!)

Sherlock inspects John who, mercifully, is unaware of the attention. He observes more grey hair around John's temples, it's a different texture, coarser than his other hairs. There are new wrinkles around his eyes and the older lines have deepened. The veins in the backs of his hands are more prominent. His skin is thinner there and in other places from the slowing collagen production. Sherlock imagines the free-radical damage accumulating in John's body at its most basic, molecular level. Oxidation.

These are new developments.

That's when it hits him with all the force of a crashing wave: John is changing. He is different from the John of yesterday, and he will be still more different tomorrow. How very not-boring! Sherlock realizes he wants to watch John evolve, wants to catalog all this new evidence that might not be apparent from day to day but makes itself known gradually over time.

The passage of time, then, and sharing your precious lot of time with another soul... that is the gift that John gives him. To let one into the other's heart at the exclusion of the rest of the world, to cherish the (admittedly, sometimes dull) moments that make up the days that make up the weeks months years. Sherlock's heart in his chest floods with blood and emotion as he remembers the rush of their new love so long ago, but now it has the impact of time behind it, all they have shared since the beginning, the memories built together, and it is stronger than ever.

(You take care of me, you accept me, you fill all my voids, at first you were a puzzle to be solved but you've surpassed that. I loved you then, yes, but I was so young and stupid and didn't know anything about relationships and my love was naive. You're not a puzzle to be solved, you are more than that so much more than that you're bigger than a mystery and you've become part of me. The best part. Here's the greatest puzzle of them all: why you love me and why you stay with me when I am not good and you are perfect.

But I am better because of you.)

Another revelation, close behind the first and, in fact, linked: Sherlock is changing, too. He has changed and will continue to do so. He has no idea where he and John will end up, but he knows they will be together. And he's comfortable with that. With John at his side, he can bear the uncertainty of their future as they grow and shape and influence each other.

He realizes that John has stopped reading and is staring at him, bemused. "You look like you've just solved a case. What are you thinking about?"

Sherlock doesn't say these things that are swirling in his head, of course; they are private thoughts and to speak them aloud right now would erase their mystique. He needs to sort them out first; and besides, it's a conversation meant for the bedroom anyway. So instead he smiles and says:

"Happy anniversary, John. "

He stretches out a long thin arm in John's direction, inviting him to take his hand. John does, and he caresses that hand, kisses it, their eyes meeting: mirror pools of intimacy.

"And many more," John promises.

Forever, Sherlock vows.