John Watson had a sudden thought about his move in with and adjustment to Sherlock Holmes. It was about acclimation. Sherlock, for all his failings with the human species' emotions and workings of the brain on a normal basis, had set up John for the perfect acclimation.
Take swimming, for example. If one were to enter a body of water, say a pond, for the first time in early summer there are two thoughts that could enter the mind. First, the pond is possibly shallow and therefore warmed easily by the sun and therefore going to be easy to swim in. Second, it's a body of water in early summer and has the possibility of still being very chilly. Instead of dipping your toes in to gauge some sort temperature from it, why not just jump right in? The acclimation to the sudden temperature difference would be much easier and much faster this way.
The same could be said for dealing with Sherlock. The slight look at the man in the lab was enough to question his…well let us say, "temperature". He could be either warm or cold. John would rather like to say he'd hoped for 'warm', and so, jumping into the Sherlock-pond head first, was the best way to go about it all. He hadn't had time to test the limits of the insanity, or 'coldness', of Sherlock so there was no questioning and therefore, possible reason to back out. He'd dove in and couldn't turn back. He'd acclimated quickly to the Sherlock-pond and now found it rather enjoyable to swim around in, even if now and again a long bit of seaweed would wrap round his ankle and make him feel as if he were to drown in any second.
The coldness was something John didn't despise, or wade back out of, as he was floating in the middle of it now. And there it was- the Sherlock-pond had, in fact, suited John Watson who may have needed the cool waters to wash away the mundane life living outside of the war.