1) John Watson worries. Sherlock knows this and dismisses it as so much nonsense.
2) John Watson also thinks too much. Sherlock knows this too and dismisses it as so much nonsense as well.
3) John Watson makes plans which are forever being broken. Sherlock knows this and tries to stop him every chance he gets. What’s the point of planning when you might find yourself running down a London alley at three in the morning pursuing a rogue taxi cab instead of sleeping in a warm bed? Holiday? Why bother when you can investigate secret government scientific bases and see exotic creatures never before seen on the shores of dear old Blighty?
4) John Watson never listens. Sherlock knows this and dismisses it as so much more nonsense. This is why John still makes plans despite Sherlock’s best efforts to disrupt them and discourage him from making more (See note 3 above).
5) John Watson believes that one day, Sherlock will no longer find him interesting. Sherlock knows this—for God’s sake, he’s been told over and over again—and dismisses it as he has dismissed the previous points. It’s all nonsense. Can you see the pattern here? For goodness sake, what it must be like to be in your funny little brains! It must be soooo boring.
6) John Watson misses Afghanistan. Sherlock knows this and doesn’t think he is odd or wrong for doing so. The army gave John purpose and structure to his life. Take that away and suddenly the world is a very different place. Captain Watson is used to issuing orders. When Captain Watson pulls rank, people jump. He has precious little opportunity for that now—automated tills in supermarkets do not recognise rank or superiority—but when he can, Sherlock engineers the opportunity.
7) John is a protector, a saviour. Sherlock knows this and has done his best to give John back that sense of purpose. He has also made sure John has had his fair share of opportunities to protect Sherlock, despite Sherlock not actually needing protection. No, really. He was never in any danger. What? Didn’t you realise? Sherlock will treat that with the same dismissal he used on the previous points. It’s all nonsense. Really, I mean, this is a man who can see off assassins and consulting criminals with delusions of grandeur, and Tongs, and naked femme fatales, and bothersome big brothers with minor—minor my arse—positions in the British Government who stick their noses into your business all the damn time. This is a man who is an expert at self defence, can deduce where the next blow is going to land before you’ve even thought about it, never mind struck it, can dispatch an attacker before John gets back with the milk and rescue said femme fatale from the Taliban, probably all in the same breath. You see, John, you weren’t the only one to invade Afghanistan, only they weren’t expecting Holmes and hadn’t prepared for his visit... So, you didn’t see any of that careful planning that meant John would be able to step in and be the big hero? As I said, your funny little brains....
The answers to points 1 through 7 are simple. John Watson needn’t worry himself. There is no way that Sherlock Holmes will ever tire of him, will ever decipher every nuance, every quirk, every mannerism that is John Watson. John Watson changes all the damn time; he evolves, he alters, he is like a chameleon. While he might not be much of an actor, John changes to suit his own environment and circumstances as easily as breathing, and he really does not need to overthink the situation, any situation. Really, there’s nothing to think about. Sherlock Holmes is not something that requires much thought. Sherlock Holmes is a situation in himself, an event, a thing to react to, not to think about. Sherlock Holmes will always react the same way to things, he will always deduce in the same meticulous manner, the same attention to detail. If anything, Sherlock Holmes ought to be worrying that John Watson might get bored with him. Yeah, like that is ever going to happen.
Sixty years from now, when John is touching 100 and Sherlock is a mere stripling at 95, when John doesn’t recognise himself from that young idealistic doctor who was going to change the world, when he doesn’t recognise himself from the army doctor who, together with his fellows, was inovating and inventing the procedures the NHS subsequently adopted as standard in trauma surgery, when he doesn’t recognise himself from the broken lonely man who took up with an unusual flatmate, Sherlock will smile patiently, he will glance at him with that oh-so-pale gaze, and say...
“That’s why you never needed to worry that I would grow bored.”