It wasn't arrogance to consider himself one of the quickest-witted men of the modern age. Arrogance would have implied that he had an elevated and undeserved opinion of his own intelligence and powers, or at the very least that his regard for himself was overstated, absurd, or offensive. (Well, if you asked John, more often than not it was offensive, but he couldn't worry about every second person getting a bee in their bonnet because he was cleverer than they were and wasn't afraid to express it.) No, people seemed to assume that because he wasn't modest that he was arrogant, a presumption that he could easily see coming from those with lesser facilities for higher-level reasoning and fragile egos. It wasn't true, of course, because he never overestimated himself - overestimating himself would only lead to failure, because of course he wouldn't meet his own expectations and then he would be vulnerable and allow his opponent to gain the upper hand - but he often grew tired of arguing with the mindless masses. So unsatisfying in the long run.
So very boring all the time.
John was different, of course. He couldn't quite place his finger on why he was different, he simply knew that he was. His intelligence was barely above average, objectively speaking; of course he had very specialized knowledge that Sherlock lacked, but in a general sense, he knew too little about too many things to be considered really well-rounded and intelligent in any one area aside from medicine. His head was full of useless facts, inane things like cosmic relations and pop culture, and they interfered with his really utilizing his mind in the proper manner. It was a pity, because Sherlock had known him to be especially keen on certain subjects, and if he only took a care with what he decided to ponder while he woolgathered, he might actually cultivate an intellect impressive enough to be really proud of.
John never took well to that suggestion. Usually the conversation devolved into finger pointing and John lording useless knowledge over Sherlock, as though that would somehow prove his superiority in something.
Being superior in mastering useless knowledge was not precisely what Sherlock would have wanted to be remembered for post-mortem, but to each their own. (Usually, this was the point where John threw a very British fit and left him to his own devices. Usually, this was Sherlock's ultimate aim in the argument anyhow, and so ah-hah! Game, set, match.)
When you looked at his CV, so to speak, he wasn't that impressive. Certainly an extraordinary doctor, but there were plenty of extraordinary doctors. He had an ease of temper and general attitude that was pleasing, largely because he seemed to take most of Sherlock's antics in stride, but that wasn't enough to wholly sell the man as being the most impressive, pleasing, satisfactory relationship that Sherlock had. Frankly, he didn't know what exactly it was about John Watson that he adored as much as he did (and admitting as much had been an uphill battle, but he'd eventually decided that putting a name to his feelings was more appropriate than ignoring them, as they tended to fester while he wasn't looking) and that was frustrating, but ultimately out of his control. What was in his control, however, was how he handled that adoration, and what he decided to do with it.
Now, Sherlock had grown accustomed to a particular way of life. That way of life included John at nearly every turn, and he found that he was happier, healthier, and generally more productive now that he had John in his life. John, too, seemed to be only improving from their continued association - after all, who did he have to thank for getting rid of that limp? Yes, quite. - both physically and spiritually. There was a robustness to his manner, a generally more happy and relaxed air that only seemed to improve as time went on (and as they weren't targeted by psychotics, though those incidents were few and far between) and that indicated to Sherlock that their relationship was a symbiotic one. In short, they improved one another by mere presence, and the longer their proximity was sustained, the greater level of improvement that came to each.
His general satisfaction at their relationship had endured for a time. Sherlock, openly disdainful of romance and all the trappings that came along with it, had convinced himself that he and John had the perfect pseudo-relationship that allowed for all the benefits of, well, married life, but didn't include any of the pesky and irritating downfalls. They were not solely responsible for one another's happiness, though they played a good part in the pursuit of it, and they did not have to answer to one another unless they wanted to. There was no obligation, there were no real demands, and yet they coexisted (largely) peacefully and harmoniously, give or take a few incidents about the refrigerator and bullets in the wall. His partnership with John was, frankly, what he'd convinced himself he would never find and therefore was exactly what he wanted, only when he had it, he began to realize how entirely insufficient it was.
Certainly, it met all his basic needs. (Aside from sex. But lust and lechery were things that could be quelled if necessary, and to date Sherlock had found them distracting and ultimately impossible to find in a long term partnership, as he could hardly put up with the same person long enough to shag them more than once, and he'd never met anyone like John prior to, well, John.) Rightfully, he had no reason to complain or even want to change the parameters of their partnership, as it was as undemanding as it was fulfilling, and Sherlock was not the sort of man who expended great effort on anything that didn't merit it. Why would he waste his energy? His time? His valuable, valuable time?
Well. Sarah, for one.
John had been mooning after Sarah for nearly as long as Sherlock had known him, and while he had been amused and indulgent at first, now it was getting on his nerves more than a little. No, their relationship wasn't progressing at any pace that worried him, but he kept an eye out and an ear to the ground just in case. The last thing he needed after achieving such a perfect example of symbiosis was to have John snatched away from him by a passably pretty woman with similar interests and little to no real excitement or stimulus in her life. Wouldn't John get bored? How utterly reprehensible. Spending one's life doing mundane things, like the shopping and watching the telly and cleaning house and cooking dinner, and oh certainly, a snuggle here and a shag there and those were all well and good, but where was the excitement? The thrill? Sarah couldn't provide him with mental stimulation, and as much as he did adore John, he knew his mind would stagnate without constant revelations and challenges, and the man could really not stand to get any duller than he already was.
(Not that Sherlock found John dull always. Or even most of the time. John seemed to be the unknowing but fully deserving subject of the John Watson Exceptionalism, though Sherlock couldn't say how long he would continue to find his best friend endearing if he was off married and raising children and puttering about someone else's flat, letting his brains rot and his limp return and all that.)
It stood to reason that Sherlock could not allow John to become more attached to Sarah than he already was. Worse, if he kept on the way he was, she might begin to have more than a passing fancy for him in return, and then Sherlock's equilibrium would be shaken and his home life disrupted and, oh, the man he adored would be off with someone else and he would be cranky and distracted and generally useless for the foreseeable future. Unacceptable. Absolutely unacceptable. So, while he wasn't generally a fan of romantic entanglements, he couldn't very well sit back and allow his John to be snatched out from under his nose while he sat back idly and couldn't be fussed to put the effort into the most basic and serviceable of relationships.
That John was drawn to him, at least minimally, was not in question. Deny it as he might, there were certain physical indicators of attraction that the truly observant would notice if they knew what to look for, and Sherlock had been documenting them on his hard drive since that first sit-down in Angelo's. He'd never seen a reason to press on the issue, however, as John had adamantly denied any desire to further their relationship past the camaraderie that they maintained, and it hadn't been a concern of Sherlock's until his own feelings ran a little rampant. Quite against his wishes, but he couldn't go back and undo the thing, and he found that he wouldn't want to even if he had the ability.
He rather liked liking John Watson. The idea of remaining with him for the rest of his days, comfortably ensconced in 221B Baker Street with all the promise of lifelong adventure and forgiveness even after truly hairbrained ideas and impressive sulks, was very appealing. Almost dangerously appealing, he might consider, except risky things had always appealed to Sherlock, and the risk implied with caring for John Watson was minor enough that he could overlook it without much to-do. Nothing really terrible could happen if his plans went a bit awry; John had forgiven him for most everything, even for having bombs strapped to his body and nearly being blown up, so a bit of seduction wasn't going to drive a wedge between them.
Very satisfied with himself, Sherlock tucked his legs up beneath him on the sofa, humming under his breath as he began to sketch out his plans for the wooing of one John Watson, MD.