I always accidentally interchange Moriarty and Mycroft’s name when I tell stories about Sherlock. One day, something hit me.

Most of the people in Shelock’s life have first or last names starting with the letter ‘M.’ We have Mycroft Holmes, James Moriarty, Charles Magnussen, Molly Hooper, Mrs. Martha Hudson, Mike Stamford, and Mary Morstan-Watson.

The rest of the people without the letter M are the people who mattered to him. We have Irene Adler, Greg Lestrade, and of course, John Watson.

The remaining non’M’ people are those who doesn’t like him or have had hard feelings to him. Sally Donovan and Philip Anderson don’t like him. Janine Hawkins for playing with her feelings.

Then what?

Sherlock is addicted to ‘M’orphine. (okay, I sound as witty and as stupid as Ryan Higa now) The last special episode shows how his addiction is affecting him, telling us that it is actually severe. And with addiction to morphine, the name of the drug sticks to his mind, thus he gets so drawn to names starting with M and some R somewhere in the names.

Sherlock is dependent to morphine, and possibly, Dissociative Identity Disorder is a side-effect. Basically, I think that everyone with the letter M in their names are nonexistent people. Idk how that would be depicted in the show, but if you’ve seen the psychological movie The Ward, you should know what I mean.

The Season 4 teaser said: “Everyone they know is under threat." 

My theory thinks of that line like this:
Everyone = every "M” person. They = non"M" people. 

Sherlock looked so haggard in the teaser and he was stressed seeing the nurses coming at him. So, I think he’s getting a treatment for his addiction. Everyone with the letter M is under threat because they are about to vanish!

And Moriarty is totally back.

He’s not dead because he can’t die. He’s an imagination! And he only dies if Sherlock gets treated, and everyone else with the letter M dies with Moriarty.

I hope I got you ‘m’indblown. Lol