Sherlock wasn't a big fan of self-analysis as he knew himself only too well. In each and every instance, he was perfectly aware of the boundaries he has for himself and the possible actions and routes he might take. He had a map of himself in his brain which showed him the best options and effective actions for each and every situation. It was too simple and not worth contemplating; but he did examine himself and update the database on a regular basis, as human beings have a tendency to change with time, though he himself wasn't that affected by it. Anyway introspection wasn't really fun when he knew all his goals and desires and when it was much more interesting to find out how other people worked, especially if they were related to puzzles of criminal nature. Yet this time he had to observe himself as he did others, in order to figure out this mystery.
Why did he do that?
He never did something like that. Ever. Just staring when there was no reason to, when there was nothing to discover, when there was nothing interesting. It was bizarre and unreasonable and pointless and so unlike himself.
So Sherlock retreated to his room and sat down on his bed and reconstructed that kitchen scene in his mind and explored what John did and what he did and what John said and what he said and what he saw and what he felt and got lost before he realised he was lost and found himself recalling John's neck in such clear detail that he found himself mesmerised by it and wasted shocking fifteen minutes like someone stoned by something too strong to be legal before he was called back to reality by John shouting to him from the living room to answer the bloody phone but he couldn't answer the phone straightaway even though it was the long-awaited call from DI Lestrade because he realised he was like that for quarter of an hour and he was physically stunned by that realisation and couldn't move.
In the end John rushed in, trailing a stream of half-formed invectives in his wake, and answered the phone.
"John Watson here."
Sherlock found himself thinking (while unconsciously deducing what Lestrade was saying from John's responses) that John should really turn his back on him so that his ne... and stopped himself. This really won't do. His approach was all tangled and confused. This was surely something that required careful and controlled systematic methodology, as evidenced by irrational confusion experienced. He needed a plan of action. He needed to establish a contained environment or at least an environment where he can apply a proper time frame for observation. He needed more data, take various samples from data acquired and analyse it to establish set patterns and draw conclusions. From now on, he told himself, he must exert strong self-control and be keenly aware that he himself is the biggest obstacle to successfully tackling this riddle of John Watson's neck.
Following such conclusions (while John was getting briefed by Lestrade about a strangler case) Sherlock formulated a plan.
In a way, it is a good thing Sherlock doesn't think or operate like a 'normal' person. If he did, surely 1) he would have felt quite foolish for going way overboard with this endeavour, and 2) would have felt quite uncomfortable for these intense feelings over his flatmate's body part.
John finished the phone call and looked at him. Sherlock was looking at him already so their eyes met, naturally. John didn't say anything. Sherlock didn't say anything. Then John frowned, the one that signified puzzlement, just for a moment. He soon discarded the frown, nodded towards the door, and said, "Shall we go?"
Oh, yes. The case.
Sherlock promptly got up from the bed and nearly rushed out the door but stopped himself in time and was rewarded by the sight of John making his way out in front of him, shaking his head slightly, and saying the following words.
"I'm quite willing to have a go at you for not answering your phone but haven't the energy for it so we will skip that part and just go, okay?"
Sherlock smiled, and followed him.
"Lead the way, John."