The Holmes' aren't exactly the most affectionate people in the world. So when little Sherlock would get upset about something, he would seek comfort in his "blanky".
Now, with John preparing to marry Mary, Sherlock is attempting to put on a strong face. He's doing such a good job, in fact, that John doesn't suspect a thing. Until he sees Sherlock carrying around the blanket...
He tries to ask him about it, but Sherlock simply denies its obvious existence. But, with years of living with Sherlock under his belt, John pretty much figures it out all by himself.
John/Sherlock, Some John/Mary, Sherlock has a comfort blanket
Now, with John preparing to marry Mary, Sherlock is attempting to put on a strong face. He's doing such a good job, in fact, that John doesn't suspect a thing. Until he sees Sherlock carrying around the blanket...
He tries to ask him about it, but Sherlock simply denies its obvious existence. But, with years of living with Sherlock under his belt, John pretty much figures it out all by himself.