Disclaimer: I own nothing Sherlock related, I'm just playing with them for amusement.
Everyone was born with a Word upon their wrist, a word that would one day lead them to their Soulmate.
The Word, while almost always simple, was almost never exactly what it meant.
For example: Say your Word read 'Tree', you might try to spend as much time out in the woods as possible and then wind up meeting your Soulmate in front of an artificial Christmas tree while out shopping for bug spray to combat all the nature you've been coming in contact with, or even in front of a painting of a tree while strolling through the city streets.
But just so you know, absolutely know, who your Soulmate is when you meet them the Universe, or whoever runs the place, deemed it that once you found your Soulmate and touched them that their name would instantly, and painfully, take the place of your Word; and you would then be bound to your Soulmate for the rest of your lives, living as long as the other lives and dying when they do, your very souls intertwining to the point that some, though rarely, can even feel what the other feels.
In the case of James 'Jim' Moriarty, whose Word was 'Pet', he had decided at a young age that having a Soulmate wasn't worth the hassle and swore off ever having one, going so far as to stop touching people least he accidentally acquire one on accident.
In the case of John Watson, whose Word was 'Pool', he had given up looking for his Soulmate after seeing how his sister treated hers; he never wanted to risk hurting someone that way, or risk being hurt himself.
And as for Sherlock... well, his Word was 'Rifle', and he was far too busy to care for something as trivial as looking for a useless Soulmate.
John rushed forward and grabbed Moriarty, yelling for Sherlock to run.
Their cheeks brushed and they broke apart with a gasp, clutching their wrists in pain.
John looked down to see his Word, his Word, fade and shift and come back saying 'James'.
Moriarty met John's eye in wonderment as he showed his own wrist and the newly formed 'John' written there, and they could both feel the inner shift as their souls intertwined, the newly opened sense that there was now someone there for them in every way.
"Oh, do shut up, Sherly. Can't you see this is a happy moment for John and I? Stop trying to ruin it for us." Jim snapped, deciding he could live with having a Soulmate if it was going to piss Sherlock off this much.
"You are NOT taking John!" Sherlock then leveled his gun at Moriarty and John, without pausing to think about it, stepped between the two.
"Sherlock, just stop."
"He's an international criminal, John, you can't go with him."
"He's my Soulmate, Sherlock, I can't NOT go with him." Moriarty came up behind John then and wrapped his arms around the smaller man's waist.
"Don't worry, Sherly, I'll take good care of him." John snorted at that.
"Like hell you will, I know what you geniuses are like, I'll end up taking care of you." Jim chuckled at that as he turned John to face him and began removing the Semtex vest and heavy winter coat.
"Really, barely together five minutes and already comparing me to Sherlock? That's a bad way to start a lifelong relationship, Johnny-boy." Jim grinned wider as he realized he was feeling John's amusement at that even though his face had remained stern.
"No, strapping me into a bomb vest and threatening to blow me and my best mate up was a bad start." The bomb hit the water with a splash and Jim pulled John into his chest, their faces only a hairs breadth apart.
"If I apologize will you let me kiss you?" Jim teased, watching as John finally realized that he could now feel what Jim was feeling, and all that that meant. It meant that they were extremely compatible for Soulmates.
"No, because you won't have meant it." Jim pouted at that and gave John the biggest set of big puppy-dog eyes that anyone had ever seen.
"Stop that. And I didn't say I wouldn't kiss you, I just said that I wouldn't do so because you falsely apologized." That was all the permission Jim needed as he leaned in and caught John's mouth in a passionate first kiss, a kiss that was rudely interrupted by a gun shot.
Looking over at Sherlock, the two new Soulmates noted that he had just shot the ceiling to gain their attention and now had a sniper's dot on his head.
"Try that again and Sebby won't hesitate to put you down." Jim quipped as he turned back to John, someone far more interesting now than a bored Sociopath.
"Sherlock, there are far easier ways to get attention, you know." John admonished before returning his attention to his James.
"You're making me feel ill." Sherlock finally said, hoping that this would be enough to pull John from the grips of his Soulmate. It couldn't be that interesting, distracting, to have one, he thought.
"Then go home and have a lie down, you've been up for a few days too long as it is." John said without moving away from his careful inspection of Jim's neck and all the lovely flavors it held.
Deciding that an audience wasn't what they needed for the rest of the night, Jim pulled back from John's delicious ear and began guiding the shorter man towards the door.
"Where are you going?"
"Hopefully somewhere with a bed. Go home, Sherlock, I'll call you when I can." With that John left the pool circled in James 'Jim' Moriarty's arms and happier than he had ever been before in his life, and happy that Jim too felt so elated to have him.
After several minutes of uncomprehending staring at the door the pair had disappeared through, Sherlock finally turned and left the pool.
The night had been a total waste, Sherlock privately decided, and it had started out so well too.
Rounding the last corner to leave the building, Sherlock slammed headlong into someone he hadn't noticed was there and caught himself with a bare hand on a bare forearm.
The sudden pain in his wrist was terrible, but it was fascinating to watch as his Word swirled and changed to a name: Sebastian.
Seb clutched his wrist while shrugging his rifle back up his shoulder, looking at it in trepidation he noticed that his Word, 'Corner', was now gone and had been replaced by the name Sherlock.
The two new Soulmates finally looked at one another and sighed in unison, neither having wanted a Soulmate and both now stuck with one.