This is all why he's now standing off to the side, awaiting his turn in a boxing match, and drinking with his attractive flatmate invalided out of the army, an unfortunate fact only because Holmes doubts he'll ever get to see him in his uniform. A sad fact indeed as there's little doubt Watson would fill it out well again eventually; the recovery he's made since he and Holmes met is already remarkable.
It was reckless and foolish to press to live with Watson, and it's dangerous to keep on doing it because in just this past year, they've managed to grow closer instead of farther apart, and Holmes can't be certain that he isn't beginning to feel something for him beyond an appreciation for a handsome face slowly getting its health back. Love is something he doesn't want to name yet, but it's undeniable that he's thought of Watson a time or two (or more than that, if he's honest) during the night.
He tries not to because he can't be sure about Watson. Watson, no doubt, is inclined toward women -- but it is something he wonders about, and he's been cautiously trying to find the answer to that question, all the while hating himself a little bit for it. He doesn't need to strike up a relationship with his flatmate; somewhere in the back of his mind is Mycroft's disapproving look, reminding him of Holmes's problems with relationships in the past. But, Holmes tells himself, it's a fact that would be worth knowing. It doesn't mean he needs to act on it. Besides, Watson probably is solely interested in women.
So that brings them to this evening. Holmes is here looking to expend some energy, most of it brought on by Watson himself, and the rest of it caused by the excitement that comes from performing for Watson because that's what this fight is. He doesn't care about boxing; he's fallen out of the sport lately, and he rarely gambles. What is important is showing off for Watson and -- well, seeing Watson's reaction.
"It's been a little while since my last match," he calls to Watson over the noise in the room. "I don't recognize my opponent's name. He probably doesn't know mine. An advantage for me," he says with a devious grin.