Disclaimer: I own nothing. All hail the great ACD.
The first time John smiled at Sherlock, Sherlock's heart almost gave out. He was, of course, spectacularly good at hiding those sorts of reactions, so John never noticed; he was too busy being amazed at Sherlock's deductions about the woman in pink. "Do you know you do that out loud?" Sherlock murmured, and immediately regretted the words as the astonished smile slipped off of John's face. His heart clenched painfully.
"Sorry, I'll shut up," John replied, the soldier's stern countenance returning with a vengeance.
"No, it's fine," Sherlock assured him. John's eyes softened a little, and Sherlock knew he's said the right thing.
The first time Sherlock noticed the crinkles around John's eyes, both of them were out of breath and high on adrenaline, having just concluded a long, adventurous chase through alleys and across rooftops. The chase had been a bust, but Sherlock found he didn't particularly mind. John cracked a grin and then, unexpectedly began to giggle, a high, childlike sound. Sherlock watched him for a moment, fascinated by the lines his laughter created. The crow's feet were tiny, barely visible-a man who didn't laugh very much. The expression looked wrong, like it had wandered onto the wrong face by mistake.
"What?" Sherlock asked, genuinely curious. He wondered at the back of his head why he was so curious about this man, what made him special—but then John shook his head, getting control over himself.
"Nothing, just—'Welcome to London,'" he quoted, his face splitting into a lovely, unfamiliar smile once again. And Sherlock chuckled along with him.
The first time John touched Sherlock when it wasn't absolutely necessary, Sherlock's skin felt as though it had caught fire. To anyone else, it would have meant absolutely nothing—just a brush on his bare forearm in the kitchen as John reached over him and his current experiment to grab the egg carton. After three months of living together, John had gotten used to (most of) Sherlock's experiments and simply cooked around him as if he were a comfortable fixture of the kitchen table itself. He didn't apologize (not that Sherlock wanted him to), didn't jerk away, didn't act like it was anything out of the ordinary, even as Sherlock saw starbursts behind his eyes and felt the burning trail over his arm that John's hand had left.
The first time Sherlock touched John when it wasn't absolutely necessary, it was out of a strange tugging sensation in his chest. He stood with John at Harry's freshly dug grave, waiting for the casket to be lowered into the ground. The priest's scratchy, droning voice dragged on and on and Sherlock's mind wandered as it was wont to do, picking out details about the people surrounding them, until he noticed that John was crying. It was a silent, subdued thing, done quietly so as not to disturb or alert anyone, but Sherlock saw because Sherlock saw everything. He felt a rush of something unknown; it pinched his chest and he felt compelled to reach out to John-was this empathy? He tried to remember what he'd read about grief, what the friends of grieving people should do in such a situation and came up with nothing; he must have deleted it some time ago, just another distraction keeping him from his work. So he reacted on instinct and cautiously closed his fingers over John's wrist. John's only reaction was to sniffle quietly, but later, on the cab ride home, he shot Sherlock a look that almost certainly meant Thank you.
The first time Sherlock held John's hand, the night had deepened into the blackest of blacks; although they were on the edge of the Thames, no light seemed to reach them. Sherlock dragged John's lifeless body out of the Thames and hauled him onto the beach, soaked up to his waist in river water. Shakily, he dropped to his knees beside John and checked for vital signs. Nothing. Panic shot through him, but he kept his mind steady and began to press on John's chest: one, two, three, four, five. Before he had the chance to lean down and breathe into John's mouth, John came to life, coughing and choking, and it took a moment for Sherlock to be able to speak for the tears clouding his throat. So he just held John's hand while he learned to breathe again, and he loved the way John's fingers curled around his and held on just as tightly.
The afternoon of the first time Sherlock kissed John, there was a massively entertaining thunderstorm going on outside. It was mid-afternoon and dark enough to be almost night, and Sherlock stared out of the window, fascinated by the colour changes and the uncharacteristic sharpness of the shadows. Suddenly, he needed to be outside, to be a part of that dark chaos. He didn't even bother slipping his feet into his Wellies (yes, he owned Wellies, bright red ones; John had laughed himself senseless when he found Sherlock struggling to shove his feet into them one soggy morning), just pounded down the stairs in a t-shirt and pyjama pants like a child might on Christmas morning.
"Sherlock, what—" John shouted after him, bewildered, but Sherlock ignored him and dashed out into the middle of the street, instantly soaked and shivering and loving every frenzied minute of it. He tipped his head toward the black clouds and shouted at them, wordlessly defiant and inexplicably delighted.
"SHERLOCK!" John screamed from the doorway, running toward him. "CAR!"
Sherlock jumped out of the way in plenty of time, but John still rushed at him, tackling him to the cool flooding ground.
"You STUPID GIT!" he shouted over a particularly loud clap of thunder, keeping Sherlock pinned with his arms, but something behind his eyes grinned, high on adrenaline. Sherlock beamed up at him, and as soon as John let go, he sat up, leaned forward, and placed his lips over John's. Neither of them were surprised when John responded with enthusiasm, reaching his hand into Sherlock's heavy curls and pulling him closer. Lightning blazed all around them; thunder clattered deafeningly, and Sherlock and John kissed and laughed breathlessly at the sky.