On the eve of his first battle, he was so nervous that his stomach rejected his dinner. Forcefully. The next evening, he didn't eat. He had made it through the day unscathed. Several of his comrades weren't so lucky. It was John Watson's first encounter with death.
The first time that a patient died under his care—the first time that he failed to save someone's life—his stomach again revolted. He hardly slept or ate for the next week.
After those experiences, he became more accustomed to Death's presence. It was inevitable for one living on the battlefield. Eventually, even the death of his friends was hardly accompanied by a feeling of nausea. That was not to say that he did not grieve their loss—indeed, he felt it keenly. But no longer did death elicit such an ardent reaction from him.
And so it was through years of war. Shedding an occasional tear, but always collecting himself and moving on. It was either that or join his fallen comrades through inattention to his surroundings. Then he took a bullet.
It didn't kill him, but he was declared unfit for duty and shipped back home. To England. Sleepy, docile England. Returning to civilian life might be his toughest battle yet. And that, perhaps, was his greatest obstacle—to him, everything was still a battle.
Then John Watson met Sherlock Holmes.
What was at first sheer amazement quickly grew into admiration, then into true friendship. He was happy to be Sherlock's friend even if the sod wouldn't admit it, and, in fact, hardly understood the concept at all. He was deeply grateful for the opportunity to work alongside and, indeed, assist, such an extraordinary man.
Then Sherlock Holmes was destroyed. His reputation was torn down piece by piece, doubt was planted in its place, and he was made somehow—John didn't know how, but it must have been a forced act—to take his own life. He watched Sherlock fall from the roof of St. Bart's hospital (the irony registered dimly in some remote corner of his mind); saw his closest friend in the world lying on the pavement amid a pool of his own blood.
At first, his mind refused to process what he was seeing. This couldn't be real. It was quite simply impossible. Then, as he pushed his way through the gathering crowd—why wouldn't they let him through? He was a bloody doctor for Heaven's sake!—and felt vainly for a pulse in the detective's wrist, he realized that this was really, truly happening. His friend's eyes stared blankly—lifelessly—at nothing.
As soon as he could stand again, he made his way back to Baker Street. He had to tell Mrs. Hudson—what? Tell her that Sherlock was...oh, God. No. Please, God, no. When John walked in, Mrs. Hudson started to say something, but faltered when she saw his expression. "What was all that about—John? What's happened? You look awful!"
He stumbled past her, hardly making it to the toilet before his stomach heaved, and for the first time in years, John Watson vomited his guts out. London wasn't supposed to be a bloody battleground! Even on their more dangerous cases, John had been there; been able to help protect Sherlock from harm. But not this time. And why not? Why couldn't he have seen that the call about Mrs. Hudson was fake? Why didn't he stay? ...why?
It wasn't supposed to end like this.