There are moments when he thinks that John is dead. That's he's died on the couch, or in their bed. In sleep, John's either restless, or he's still. His breath will become so shallow that the rise and fall of his chest will be all but invisible under his jumpers and cardis. Sherlock has to resist the temptation to hold a mirror above John's mouth.
It's not rational. John's in excellent health. There's no reason to think that he will suddenly die with no warning, like an infant in a cradle.
The one time Sherlock gave in to his fear and pressed his ear to John's chest, John woke and was not pleased. Sherlock tried to explain but he's not good at explaining what he feels inside, only what he thinks about things outside so it came out wrong, and John was even more annoyed.
They both know that it's more likely that John will die in battle alongside Sherlock than unexpectedly at forty, or quietly in his sleep at ninety, but since Sherlock dreams that scenario too, it's hardly a comfort.
After John wakes for the fifth time with Sherlock staring at him he says, "Alright, out with it. What are you thinking?"
"That I don't know why you put up with me."
John smiles, "I worry about you too."