John Watson's life revolves around Sherlock Holmes.
It is a simple, irrefutable fact.
Ever since the day he agreed to the flat share, it had become concrete. It was like a chemical reaction: these two elements, mixed under these circumstances, could only yield this result. It was in John's personality, just as much as in Sherlock's. Yes, they could function separately, but why bother when together they were more efficient? Like a catalyst to the crime-solving reaction that was the mixing of Sherlock and data, John sped up the process, making it easier on everyone.
His life, from now on, was linked with Sherlock's: a solution, rather than a mixture. Something that cannot be separated by physical means. Yes, they had their separate components- a family member John has never met, a mate to drink with every now and again- but really, they are bonded. Inseparable. Linked. A new compound.
His life will be an atom; Sherlock, its nucleus. At the center, holding everything together, giving purpose and form.
Sherlock Holmes is the center of John Watson's life, from now on into the future, until John dies. Because John will die; it's human nature. It's chemistry. But even if Sherlock dies first, John's life will still revolve around him. The properties of an element can't be changed; that's what makes an element what it is. That's what makes John, John.