Before John, Sherlock used to come over all the time. At random hours of the day, no matter if Greg was home or not, he just came over.
It had started when Sherlock first tried to sober up. Greg's wife hadn't quite liked it, because Sherlock tended to leave a mess, but she had allowed it, never complaining about it. But Greg had known she didn't like it, just by looking at her pursed lips and glaring eyes. Greg shrugged whenever those glaring eyes landed on him, Sherlock had needed all the support he could get, and if it meant he'd stop using, Greg would ignore his wife. Eventually, she left and Sherlock kept coming over.
In the beginning he came over almost solely in the evenings, right before Greg and his wife would prepare to go to bed, and slept on their couch. He would leave in the mornings without saying a word to either Greg or his wife. Gradually, over time, he'd sometimes stay for breakfast. Still not speaking, and glaring right back at Greg's wife. Greg would try to cut the tense silence with stupid jokes that would make Sherlock grin at him and would make his wife roll her eyes.
After about 6 months Sherlock started coming over earlier, sitting left on the couch, with Greg next to him and the wife on the other end. The wife would just huff when Sherlock would sit down and go to bed, while Greg stayed for a bit longer, just watching stupid television shows with Sherlock. At the end off the evening, when Greg would get up from the couch to go to bed, Sherlock would squeeze his hand real quick before laying himself down on the couch to go to sleep. It always made Greg smile.
It didn't happen often that Sherlock would come over, at most once a week, mostly on the weekend when Greg wouldn't have to get up in the morning to go to work. After the wife left it happened more, 3 or 4 times a week even. Greg was glad it did, he grew quite lonely whenever Sherlock wasn't there in the evenings.
Greg had given Sherlock a key as soon as the wife left. Sherlock had just looked at him and nodded. Greg still didn't know what the nod meant, but Sherlock had started coming over for dinner after that. They'd work around each other in Greg's tiny kitchen to prepare the dinner and Sherlock would actually eat it too. Greg was glad he did, he was always worried that the younger man didn't eat often enough and he suspected Sherlock would only eat when they ate dinner together.
Soon Sherlock came over during the day too, and Greg wasn't always home when he did, but Sherlock left traces of his presence. A used spoon where there hadn't been one earlier in the sink, a used mug next to the coffee maker, his coat that he had left laying over the couch. Greg would smile fondly and would feel a bit less lonely coming home to an empty home.
Then John came along and Sherlock didn't come over as often any more. If he came over two nights a week it was a lot, and Greg missed him. They didn't talk much when Sherlock was there, but they didn't need to talk to understand each other. Greg missed him a lot. He dreaded coming home without reminders of Sherlocks presence to welcome him. He ate less himself, not really in the mood to cook when it was just for himself now.
After a few months Sherlock stopped coming over altogether. Greg spend a lot of time at work then, often sleeping on the ratty couch in his office, groaning in the mornings because it really killed his back. But he couldn't face going home to an empty house.
Sherlock would frown whenever he saw Greg stretching his back out on crime scenes, but he'd never comment and Greg would avoid eye-contact.
Greg knew when he had been replaced. And by God, he missed Sherlock more than he missed his wife.