I believe this is a good quote to symbolize the tragedy of PTSD. The author deals with this a lot in The Things They Carried. But the symptom that most people know is that a person who has suffered through something so horrible that it messed up his mind has trouble conforming to peace and normality again. That's the feeling of being alone in the midst of others.

In this case, a soldier has been given a chance to taste that peace again and it's so sweet it hurts. The reason for this is that when a man is sent to war, most often the first fear is that no one could possibly get used to it. All the fear, the madness, the danger, the long marches through enemy territory. But it happens nonetheless, to most soldiers at least. They become weirdly comfortable in it, like settling down in the calmness in the eye of the storm. Being in constant mortal danger becomes the new norm and replaces their previous lives.

The thing is, however, that they still remember the world they lived in before. And it's well documented that a lot of soldiers have felt the shock that this quote describes. For months, they've cursed the war, hoping to get out of it, longing for that peace. Then, just like the soldier in question, they find that it is so good it hurts. That's because they can no longer enjoy the peace. The switch back from their distorted normality is much harder to achieve. But they still remember all the things they left behind. They know that it has been taken away from them. Not in a physical sense, that is. Their homes are mostly still there, so are their loved ones. But they've lost the ability to connect to any of it.

In the case of this soldier, the realization brings anger. He now wants to go back to the war, to the life he can still relate to. And he wants to hurt the peace because he fears that he'll never be able to enjoy it again.

One of the things that is important to note about O'Brien's work is that he doesn't always make it clear exactly what anything is supposed to mean, he leaves that work largely up to the reader.  In this case, I always read it as something like a commentary on what the conditions of war can do to someone.  So much of what O'Brien writes about leading up to that little story is the desperate search for ways out of the death and terror and boredom and dismay and burdens of war.  This guy found it.  He could just sleep with his nurse all day and be free of it.

But something about the lack of hurt and pain drove him back to his unit.  He had to find some way of getting back at all that peace because it felt wrong, he had to break it because he was perhaps so conditioned to being in that condition.