Sherlock would never admit it. Joan would never admit it. But everybody around them knew it to be true. They were hopelessly in love with each other. Some may laugh at such a notion, especially after seeing the way Sherlock treats Joan. There were but a few who knew this was to stop himself from feeling anything other than a professional relationship towards her. But people who knew Sherlock before his addictions such as Captain Gregson knew better than to just read into what Sherlock wanted him to see.
Gregson remembered meeting Irene, at the time he had no idea that she would turn out to be such an important part of Sherlock's life only that she was a business women who was Sherlock's friend. He should have seen it coming the way he praised her… the way he looked past all her arrogance and saw only beauty and skill and passion within her. Gregson saw her beauty and possibly everything Sherlock did but he knew it would not last… she was like fire, damaging everything in their path and Sherlock got in her path and when news reached him that Sherlock was in rehab and that his fiancée (much to his surprise) was dead he knew not to push Sherlock but to give him what little he could to make sure he was able to stay clean and get the help he needed.
So when he introduced Watson to him, he knew straight off she was something special, not by anything he said about her, just merely the fact that she could take all his half witty insults and turn them back towards him in a way that was both humorous and endearing. Sherlock is an impossible man, he knows full well. But over the course of the six weeks Watson was with him. Those weeks where she lived with him he had never seen Sherlock so rearing to go. There was moment when Gregson would catch them interacting in the prescient, her stealing his crisps while insulting him in some way and he would steal them back and argue back with her. They both held smiles then, such smiles he'd only ever seen on his ex-wife on the day he got married.
When the six weeks ended and Joan left, Sherlock almost fell back into his old ways… He had a needle and everything he could possibly need to stop himself from thinking about Irene, about Watson right next to him. As Gregson unlocked the door and barged in, thinking he'd find Sherlock ODed on the floor he saw Watson hugging a crying Sherlock on the floor of the spacious living room. Watson gave him that kind of caring smile that thanked him for being there for Sherlock. Gregson left then and he knew Watson would be around longer than even he wanted to believe Sherlock could deal with.
Sherlock was still the same impossible man, with enemies even Gregson didn't want to believe exists, he was still arrogant and frank with people. The very thought of Watson over the years of knowing them both was a way Gregson had kept Sherlock away from drugs and danger and back into her arms safely. But around her is when he noticed it most, how close he would stand to her, especially if any threat was still present at any given time. How Sherlock smiled more and joked more at crime scenes and it the most inappropriate of situations. But most of all it was in the way Sherlock looked at her… like she was more fragile than he was. Like she was a goddess in his presence and like most things he couldn't explain… he pushed her away to the best of his abilities, but over the years of Joan helping out on cases Gregson knew he had only the tip of the iceberg compared to what Joan knew and Understood about the man that is… Sherlock Holmes.