Summary: Sherlock Holmes has never been, nor will likely ever be a romantic man. That didn't matter, though, because his relationship with John was so much more.
Sherlock Holmes could never be in a romantic relationship, it would have been far too dull and trite of a thought to even cross his mind. Sure, he'd known from that day at the pool how much he actually needed John in his life. He may never show it, at least not very well, but John had quickly escalated to being one the most important people in Sherlock's life, one of the few that mattered. The two of them were far too proud to admit such feelings, if they ever even had them. There would be no point, anyway. What they had was so much better and much more complex than that of a couple. Neither was aware of the matter on a conscious level, but it was there nonetheless.
To label their relationship as 'romantic' would be insufficient, since they are so much more than that. No one could understand each other as much as these two, and no one could respectively tolerate or be tolerated by them for long enough to become so essential to their lives. Sure, some could come close, but none could ever compare. Sherlock and John in essence became two parts of one whole; Sherlock became more human and was taught limits through John, and John's then-dull world was given life and enjoyment through Sherlock.
John would never know just how important he was to Sherlock Holmes. He might get an idea by comparing Sherlock's interactions to others around them and from hearing about how he's changed, but he'll never witness it first-hand. He'll never really know. While they may have originally existed separately, there could no longer be one without the other, not really. They could be separate, but they wouldn't really be themselves anymore. They were only 'complete' when they were with the other; when separate, there was always something unidentifiable missing. They may not fully realize it themselves, but those around them could tell. As outsiders looking in, they could see the changes in Sherlock's behavior, how much more human he was becoming. He could never be fully brought down to their level, to do such would be to take away his very nature, but at the very least John Watson taught him the very basics of acceptable social interaction while never changing Sherlock as a person.
Few people would ever realize just how much of an impact Sherlock had on John's life as well. Sure, they understood how important Sherlock had become to John- the person he had eventually placed first in his life, valued above all the rest- but they had never truly seen him at his lowest, and so had never seen how far he had come even minutes after meeting the other. His life had been given new meaning through the excitement and uniqueness of a life near Sherlock. No one could ever have filled that void quite as well as the younger Holmes brother, and no one could ever compare. What may have once seemed exciting for John could now fall short in comparison to Sherlock's antics or the cases they went on together.
They don't need romance. They're just fine without it, and no romantic relationship could ever compare to what they already have. Maybe that's the reason why all of John's attempted relationships have all failed. Apart from the obvious issues, he's always subconsciously comparing them to his interactions with Sherlock, and they always seem far more uneventful and unfulfilling. Sherlock set the bar that no one could ever hope to overcome, and no one would ever take his place.
This is why Sherlock would return to John as soon as possible, and why John, despite all odds, would always believe in him. He would always believe that Sherlock would find a way back to him, and he would wait.
This isn't to say that I hate the pairing or to bash on anyone who writes Johnlock. On the contrary, I quite enjoy reading it at times. However, there is a difference between canon and fanfiction, and I firmly believe that a romantic relationship between the two should stay in the latter. I feel that a romantic relationship would actually be a step down from them, and that it would almost seem degrading to their already-fantastic relationship to put them in the same position as all the other couples in the world.
Anyway, thanks for reading, and feel free to tell me your opinions (in a calm and insult-free manner). Any reviews would be welcome, especially constructive criticism. I'm a little unsure on whether or not I managed to keep it in a consistent tense throughout the entire fic or if I kept bouncing between the past and present at the wrong times, so let me know if that feels 'off', or if it seems disjointed at all.