At first, he’d chocked it up to another one of his moods, a fleeting fixation brought on by their perplexing new case, but when it continued, incessantly, day after day, something inside John began to fester.

Edited to add: there's a prequel.
"We played cards in the living room. The deck was on the end table, and a few cards were missing, but I found them under the sofa. I'm not sure what we played, but the two cards I found were both jacks."
"It was 'Go Fish.'" John blinked at him.
"Are you serious?" he asked incredulously. "Jesus, how old are we?"
"By your insistence."
"No…"
"I wanted to play bridge."

I would have chosen Go Fish too, under the circumstances. Maybe it's just that I'm not as smart as Sherlock, but I don't see how John and Sherlock can play a four player game with jsut the two of them.