The Game
Sherlock was BORED. Life was boring, people were boring, though to give John some credit, he was the least boring of the lot. Why couldn't the crime lords of London hurry up and give him a case, something to do. And there was one very specific consulting criminal that Sherlock had every mind to simply call and ask for a murder to solve. James Moriarty, consulting criminal. His network of informants, gunmen, and hackers (though he was quite an accomplished one himself) was as intricate as the branch system of a gnarled tree, a tangled spider's web. And, as Sherlock had put it before, Jim knew how every strand of that web danced, how each branch was anchored to the trunk. Jim Moriarty always came up with the best, most satisfyingly mind-boggling cases for Sherlock to solve. Games, he called them. And with John being so disapproving of the morbid way that Sherlock loved a good homicide case, it wasn't exactly easy to have Jim get the ball rolling.
That was what made Moriarty and Sherlock different from everyone else, unless you're counting Mycroft. The brains of Sherlock Holmes and James Moriarty were not, could not, be satisfied with the daily grind of a "normal lifestyle". Without satisfactory stimulus, their minds would only descend further into what "normal" people classified as insanity. Not that people didn't already call them insane, it's just that Jim was a criminal mastermind, and acted like it, and Sherlock didn't much care what people thought about him. Jim was the living embodiment of the word insane, while Sherlock preferred a term he coined himself, "high functioning sociopath." However, both of these unordinary men went about sating their thirst for excitement in very different ways. Moriarty, in committing extremely creative and horrifying crimes; Sherlock in solving them. To quote his "archenemy", Sherlock was on the side of the angels. And, every fairytale needs a good old fashioned villain. The problem was, Jim's cases were like a drug. They were simply so much more interesting than nearly all the other cases Sherlock had done. Sherlock knew how obsessed Moriarty was with "playing The Game", but somewhere deep inside him, Sherlock could tell that he was too. It's all about playing The Game. Moriarty was drawing him in, giving him little tastes of what it was like to be challenged, and Sherlock would be lying if he said he didn't love it.
Sherlock liked to consider himself an emotionless machine, but he also knew, as Jim put it, "...That's not quite true." He was bored in the absence of cases, he was excited and energetic when on a case, and he also felt, though he hated to admit it, a connection with the people closest to him. With John. And John, though also spurring happy feelings, also caused Sherlock to feel guilt. Because Sherlock knew that John could handle him being ecstatic when faced with a murder, as they were solving it, but….it was a completely separate thing to encourage a consulting criminal to cause such murders to take place. So, Sherlock was guilty. He cared about John, and wanted to make him proud to be his friend, but the monotony of average life was so draining. Jim could get rid of that. And Sherlock knew that Jim Moriarty knew what he was doing, because as much as Sherlock needed his brain to be worked, Jim Moriarty was getting just as much out of trying to trick him. They were the same. They both hated the toil of day-to-day life, interacting with boring people, and strangely, feeling as if they were the only ones like them in the world. But when James Moriarty started The Game, Sherlock knew that there was nothing he could do to stop himself from escaping boredom and playing The Game while the world burned.
And as soon as Jim left the apple with the letters, "I O U" carved into it at 221B, Sherlock also knew that he didn't much care how The Game ended. He could see Jim holding back a manic grin as he whispered, "Because I owe you a fall, Sherlock. I. Owe. You." As much as Sherlock loved running the streets of London with John, solving more trivial, less complicated cases that only barely stimulated his mind, he was always drawn back to The Game. And somehow, Sherlock knew that neither him nor Jim Moriarty would make it out alive.