Their love was explosive.
The kind where it was obvious that they couldn't live without one another, but also if they stayed together someone would die. The most likely scenario would be that they would take each other out. Sherlock had a temper, but so did John. All their friends agreed that at any moment their romance would combust, and leave everyone with third degree burns. Sherlock felt that he was too young, that he didn't have enough experience to be in a committed relationship; he was too damaged from the trauma he had faced early in his life. John didn't know how to deal with emotions well, choosing to clam up instead of expressing them.
Like I said, they were clearly in love. The kind of love that destroys everything in its path, thats so passionate it hurts. The kind of love people don't always recover from.
It wasn't always painful. John could be incredibly romantic, and Sherlock knew just what to say to apologize. They promised that the next fight wouldn't be as bad. They discussed ways to make living with one another easier. They tried. But things don't always work out for the best, no matter how hard you try.
When John first saw Sherlock, he thought that he was the prettiest thing he had ever seen. He was taken aback, in awe, it was love at first sight. This isn't how the love story starts, its just how they meet. And it is most certainly not how it ends.
Sherlock was younger than John, still in school. This made John do a double take. It would be a few years before John officially asked him out. Sherlock dropped out of school; there had been an accident that he never really recovered from. Sherlock felt abandoned, like trash. Despised by his own family. Worthless.
John proved that he wasn't. He, quite literally, swept Sherlock off his feet. They moved in together. Everything was amazing. They got along well, John would go to work, Sherlock would do his experiments, sometimes he would go to Barts. Then the fights started. Turns out John had a bit of a possessive streak that Sherlock couldn't deal with, and, well, Sherlock had command of a vocabulary that could cut like a knife.
Sherlock could be vindictive, going out, drinking, flirting with other men, just to spark a reaction. He would go home and flaunt his indiscretion. And then he would be shocked when John became enraged. John could be just as bad, lashing out verbally, pointing out Sherlock's social ineptitude when he got angry, or bringing up things from past arguments. Sometimes John would throw things, and others Sherlock would wave around weapons, or threaten to harm himself.
Now, there was never any physical abuse. There was lots of shouting, and the dishware as well as various other fragile things never lasted more than two weeks, but they never physically harmed each other. No, verbal and mental abuse was their chosen form of venting.
John wasn't exactly a saint. He was commandeering, and hard to relate to. He seemed like he couldn't be bothered to make an effort. He would go to work, come back, eat, go to sleep, and forget about Sherlock inbetween. But when he realised what he was doing wrong, he would bring back sweet things, like a book that he had seen Sherlock eyeing at the bookstore, or ingredients necessary for various experiments. And milk. He always brought back milk. You see, John wasn't as unobservant as Sherlock liked to make him out to be.
Their fights could be terrifying. The neighbors called the cops more often than not, fearing for the safety of the couple. Every once and awhile one of them would threaten to leave. This never went over well, almost always ending with broken walls or glass, and various threats of physical harm that were never followed through with. The cycle would start over, and things would be great for a while. They cooperated. John would make tea in the morning, and remember to kiss Sherlock goodbye. Sherlock would stay away from the bars, and ensure that whatever experiment he was currently working on was non disruptive.
But then Sherlock would forget, and accidently set fire to something, or John would stay late at work one too many nights. Lighting the apartment on fire by accident almost always got the same reaction out of John, stubborn silence and a refusal to even talk to Sherlock. Then angry passive aggressive messages, and then full on shouting. When it was Sherlock's fault, the fight almost always ended quickly, but the tension that built before the explosion happened could last for weeks. Three weeks was the record for the amount of time that John remained silent, his anger building until he finally snapped when Sherlock came back drunk and depressed, and itching for an argument. That was a particularly violent outburst (their kitchen table didn't last the altercation, or the knives John thrust into it). John was taken into custody by the police for a few hours; the neighbors had said something about threats to burn the building down.
It was after a particularly nasty fight. Sherlock wasn't sure what the fight was about exactly, but he knew that it was something stupid, and that John felt responsible. Sherlock knew this because it happened at the shops while John had been getting something as an apology.
It was an accident really. It wasn't meant to happen. They told Sherlock that it was a robbery gone wrong, that John was a hero. He had stepped in front of...no he can't.
All Sherlock knows is that John is gone. And that he had never gotten to apologize, and now he never would.