Greatest Fears
Sherlock didn't used to have fears. He used to worry about no-one; he never used to have people worry about if he dies. Mycroft cared a little, but mostly he felt obligated to watch over Sherlock. Then John came along, a short, seemingly insignificant army doctor with a psychosomatic limp. But from the moment Sherlock met him, he had a feeling John was different. And he was right. John worries about him, makes him eat and sleep, and has saved his life countless times in countless ways.
He has also put up with Sherlock willingly (unlike Mycroft) for longer than anyone Sherlock can remember. John makes Sherlock laugh, smile, yell, and be more human. But with being more human comes fears. He tries to ignore it every time John storm's off after Sherlock did something or another. He tries not to worry about weather John will come back.
John is Sherlock's greatest fear. Somehow the great Sherlock Holmes had unknowingly become dependant on John Watson, and that scared Sherlock. No one stayed with Sherlock forever. They would always leave him; this was a truth Sherlock had come to accept. Than John came along, and no matter how hard Sherlock tried, John had somehow become the most important part of his life, almost as important as the crimes and puzzles. Sherlock Holmes's greatest fear was John will one day get angry at Sherlock or sick and tired of dealing with him, storm out the door, and never return.
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After the war, normalcy was John Watson's worst enemy. He didn't know what he was missing in his life until he met Sherlock. Sherlock was different and dangerous. John had killed a man for him after knowing him for less than a day. Sherlock thought everyone was stupid and boring, and he made sure everyone knew how much smarter he is than everyone else.
Sherlock Holmes was impossible, stubborn, insulting, and very arrogant, but after he spent more time with Sherlock, Sherlock let him see his emotions. John had become dependant on Sherlock. This terrified John. Sometimes he would let his mind wonder, Why me? I'm boring, mundane, an idiot. What if one day he gets bored of me, if he decides to kick me out? And this thought killed John a little every time he let himself think it. Because this was John's greatest fear, that Sherlock would get bored of him. John doesn't think he would survive that.