Sherlock knew that he was not nice. He couldn't be, because every Christmas he was on the naughty list. That was the only thing that could explain why he never got what he truly wanted for Christmas.
He did get presents for Christmas, ones that he liked very much. There were toy trains, and a chemistry set, and his new bug collecting kit. And there were other good things, like father's fairylight bow tie and going to France to visit Grandmama, and tasty pastries!
But when the day was over and the wrapping paper was cleared away, he knew that he must not have been forgiven for dissecting a dead pigeon on the dining room table, or setting fire to the potting shed, or for the myriad of other accidents that had occurred throughout the year because he was too clever, and too curious, and too unclear of the social cues around him to fit inside them. He must never have quite got his name struck off the naughty list, because his wish never did seem to come true.
After Redbeard was put down, he didn't even try any more. He ignored the gifts, and frowned at the decorations, walking past his father's new singing socks without comment, and refusing the pastries, even though they were warm from the oven. Without a child to appreciate it all, the Christmas traditions eventually died away, and Christmas became a time for his parents to take ocean cruises, while Mycroft used the peace and quiet to get ahead on his work. Sherlock huffed at the songs and the decorations and at the very idea that there was some magical creature that could grant wishes, although there was still a part of him that remembered how fun it had once been.
As an adult, Christmas was a time to plan involved experiments, or to take long walks down the riverside. It was a time to be alone so that no one else could see the disappointment in his eyes when the clock struck twelve and another year passed without him getting what he had so dearly hoped for. So when, while playing his violin one Christmas, he turned from the window to see John sitting in his chair smiling at him, he had to blink his eyes to keep from crying.
This year, he must have done something nice. What else would explain it? Because he had finally got the gift that he had wished for all of those years ago.
Someone to talk to and go on adventures with. Someone who understood him and appreciated him. Someone to love.
Sherlock had finally got a friend.