Sherlock misses small things.
(John lives for small things.)
Sherlock misses the way John smiles at particularly gruesome crime scenes when he thinks no one is looking, and the satisfied twinkle in John's eyes whenever he 'accidentally' hurts someone. Sherlock misses how John is completely oblivious to pain, unless it's being inflicted on him (in which case John has to remind himself that real people do, in fact, feel pain).
Sherlock is completely, blissfully oblivious to the fact that the times John goes missing for hours on end just happen to coincidentally line up perfectly with especially violent crimes.
Sherlock misses how completely fixated John is on. How completely and utterly obsessed the small, meager looking man is with the egotistical detective.
In short, Sherlock misses everything important, because if he was paying attention (and he wasn't too busy falling slowly in love with John), then he would've noticed long ago that his beloved blogger was a complete and utter psychopath.
You'd never guess that John is a psychopath.
(Even the great Sherlock Holmes couldn't figure it out - how could you?)
To the ordinary eye, John looks like nothing more than a simple, slightly chubby, adorable man with a kindly disposition and a ridiculous flatmate.
(Sherlock is - in this respect - tediously ordinary.)
John has even the great Sherlock Holmes fooled. He couldn't be happier, watching that fabulous man dance around his (or should he say, "Moriarty's" crimes).
Jim is very good at his job.
(Moriarty. The Art of Death. How appropriate and obvious. It's a fortunate miracle Sherlock doesn't put it together in time.)
Moriarty promises to burn the heart out of Sherlock Holmes. John Watson is Sherlock Holmes' heart. John Watson is Moriarty. It couldn't be more perfect.
John is very possessive.
Sometimes - when they're out in public or interviewing someone for a case - John wants to hoard Sherlock like a precious treasure. John wants to own Sherlock. (But to possess him is not enough, oh no, John needs to be him.)
When women look at Sherlock that way, John invites them out on a date and treats them with the delicate respect they deserve. When men look at Sherlock in that way, John usually finds a clever and quiet way to dispose of them.
He almost has Molly killed. Almost. Instead, he has Jim play with her a bit; string her along and trick her into falling in love with him. It makes the reveal so much sweeter.
Sherlock's face at the pool is priceless.
(John imagines his frantic brain trying to put everything together, scrambling to figure out what he missed.)
It gives John infinite pleasure to think that Sherlock knew - for a few seconds, at least - who John rearly was.
And the next second Jim steps into the spotlight and Sherlock breathes a sigh of relief and John feels empty again.
"No one gets to me... And no one ever will."
(Those words couldn't be more true.)
John is very good at being John. But he's even better at being Moriarty.