Sometimes John forgets that Sherlock is just a human.
He forgets that his heart beats, his lungs breathe, his blood pumps and his eyes see. He forgets that it's the same kind of heart that pumps blood around his body as everyone else's; his lungs are no different from John's; his blood is red as any other and his eyes are merely windows into the same brain as the rest of the world.
He sleeps (sometimes), eats (rarely) and craves human affection (albeit in the form of corpses or the skull, though it has the same effect) just like we do. He hurts (and complains to John about it incessantly), he laughs (well, it's more of a snigger), he cries (or John expects so, he's never seen it with his own eyes outside of the fall, but let's not get into that). He talks in his sleep (French, usually – although he'd never admit it), sings to himself when he thinks no one's watching (badly) and is remarkably vain (he owns far too many hair products).
Sometimes, John forgets that Sherlock is just a human.
He forgets that eyes might one day stop seeing, that his blood might not pump around his body someday, that he'll eventually take his last breath. He forgets that one day his heart just might stop beating.