Sherlock Holmes is a man who likes mysteries. He is addicted to solving them, and is always on the lookout for them as this addiction needs to be fed. When unfed, he has no choice but to plunge into a different kind of addiction in order to ignore the unsatisfied longings.
Mysteries exquisite, of exceedingly high quality is what he seeks. Mundane ones of everyday sort will just exasperate and bore him. To keep Sherlock's interest unflagging, it needs to be complex, exceptional, intricate, and unexpected. It should be deducible, logical and sensible. He collects them only to solve them, then deletes them afterwards. Solved mysteries are boring though they do turn into a kind of trophy, a mention in his long list of achievements, and becomes his not-so-secret source of pride. (For he does take pride in them; they are evidences of his abilities after all.) His long track record of solving them is what affirms his belief in the world as a logical place where there are laws and rules that are adhered to, even if they may somewhat be bent and screwed. It renews his belief in humanity; that people can be known, explained, and understood. Without it he will stop seeing the point of it all. For what is an illogical world to someone who only craves and thrives on solid logic? He dislikes true irrationality. It is meaningless and pointless, and unsettles him somewhat. There has to be an explanation for everything.
Of himself and how his mind works, Sherlock is well aware. He gets bored by predictable and cannot abide them. At the same time he craves logic that overrules everything, a motive that can be picked up in anything and everything. He wants an ordered chaos, decipherable riddles, logical puzzles - which would seem ordered, decipherable and logical to no one but he himself can make sense of if he pursues it.
And he is now faced with the strange riddle of John Watson's neck.
Rather, the back of John Watson's neck and his fascination with it.
To be exact, the enigma of why in the world is he, Sherlock Holmes, so intrigued by that part of John's body that he can't help looking at it and wants to gather more data regarding it first hand.
Sherlock wasn't even aware of it before - he of course knew John's neck had a back and it existed, but he hadn't really properly perceived it, really looked at it before. He only became fully aware of it after living with John for about four months. This is not due to lack of attention or oversight on his part but more to do with circumstantial factors (the most influential one being the season; they met in winter and John always covered/wrapped/hid that part of his body with his shirt collar/scarf/jacket collar turned up against cold).
So when the weather turned warm and John began to spend more time lounging about in his dressing gown in the living room, rather than getting changed into something warmer as soon as he can (which is right after the shower), Sherlock was surprised to find that while the back of John's neck did indeed look exactly like the simulated version he had constructed from calculating available visual data, it didn't. He doesn't know how to express himself as he does not know what this is. It was not simply the back of John Watson's neck, a part of his body which was there as it should be, all anatomically correct. It had a strange effect on him.
He couldn't help staring at it.
And he doesn't know why - at all.