No one had ever laughed with him before.
Sure, they'd laughed at him, plenty of times, hundreds, probably thousands of times, more times than he cared to count. People had been doing it for years, so long he'd long since given up trying., not that he'd ever been good at it. But no-one had ever laughed with him, especially when he hadn't even meant to make a joke.
"Nothing, just...welcome to London." A wry grin and high, adrenaline filled chuckle.
He hadn't expected it. It hadn't been a joke in his head, just a way to confuse the man and hopefully stop him reporting them. But John had laughed.
John had laughed... and something warm flared inside his chest. Just for a second, before curling up again, but it stopped him, just for a second his brain stopped functioning as the warm feeling spread, close to the drug high but so much better.
He'd never made anyone laugh before.
And so he'd tried again when they'd gotten home.
You invaded Afghanistan.
And John had laughed again, harder this time, and grinned like a maniac, his leg quite forgotten. That tightly wound, most buried but oh, so warm thing, had uncoiled that little bit more, bright and comforting, like he couldn't remember feeling.
Well that wasn't quite true- it had uncurled a little with Mrs Hudson, who called him "dear" and put up with far more from him than anyone else had. How she brought him tea and bothered him about his health and experiments till he wanted to hit something, but at least she bothered to notice.
And it had shifted comfortably when Lestrade had pulled him from the gutter with a wry grin and a helping hand. And all those times afterwards when he had relapsed, and Lestrade had sat with him, sometimes all night, waiting for him to come down. Who had always believed him when he tried again, and again and again. Who had helped him find his work, the thing, the sanity and stability he clung to so hard. It had even happened with Molly once, when she had forgiven him, yet again after a particularly vicious rejoinder. How she kept on forgiving him.
And so the high functioning sociopath discovered he liked the warm feeling. In a world where everything is just a set of connections, and the connections are the only things holding everything together, it's nice to know there is something else. Something comfortable that sits in his chest, which makes him warm when the doctor laughs.
And suddenly it becomes important that the doctor keeps on laughing. Maybe there was something to this "friends" business after all.