Human Interaction
It amused John quite a bit how most of the time Sherlock very obviously didn't care about peoples opinions of him; unless of course the were complimenting him in which case he seemed to turn into a flattered school girl. However on the rare occasion he acts self consciously and this occasion was certainly one of those times. People generally thought of John Watson as a man of average intelligence, but not really anything to note on except the amazing ability to put up with the worlds only consulting detective. On the contrary John Watson was a rather intelligent man but his brainpower was hugely overshadowed by Sherlock Holmes. Perhaps it was the fact that he didn't mind this that was part of the reason he could withstand living with Sherlock.
It always astounded John Watson how simple and obvious the mans deductions were when explained to him and so he strives to be able to have even a fraction of his power. So John Watson did become more observant which was why one evening when Sherlock casually asked him to accompany him to a violin concert he knew that Sherlock wasn't just acting on a whim as he would like John to think but had actually been planning this outing for ages as told by the flyer that had been lying on the kitchen table a couple of months ago and since then had lived folded up in his coat pocket.
That was how John came to be seated next to Sherlock at a violin concert where he felt really rather out of place. It certainly wasn't bad, however violin music wasn't really his thing, unless it was Sherlock playing it with the one condition that it wasn't at four in the morning.
As the evening progressed John found his mind wandering. It wandered to many placed but usually kept finding it's way back to the man sitting next to him. It was odd how he looked so calm, relaxed and distant. Not distant in the unreachable case solving way, but as if he were off in his own world. But what struck him most was that he looked happy. Not happy like when he had just found out about a triple murder or even when he had just solved a triple murder, but more like when we went to that chinese after our first case.
Strangely, while thinking about Sherlock Holmes John forgot that yes this was Sherlock Holmes and was soon greeted by a questioning look from the detective in return to his staring.
John just smiled at him. Sherlock smiled back. They turned to face forward again and very suddenly Sherlock reached out to grab Johns hand.
At this point many thoughts passed through Johns mind such as: What the hell? I'm not gay. I really should push him away soon. It's probably too late to do that now. But in the end he decided that he would have plenty of time for an internal battle later on and he might as well enjoy the current situation while it lasted as he would have to deal with whatever effects this had later anyhow.