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.