He absolutely didn't want to go back to the war. He hates being here.
But he has a job to do. Two jobs: the pointless, sick one they're all here for, and the one he actually cares about. Wait and march, kill and die, as though some scrap of muddy ground is worth all these men's time, let alone their deaths. Make sure David makes it home in one piece.
(Or, at least, that he doesn't die alone and terrified....
Makes it home in one piece. Can't imagine any other outcome. Can't let his thoughts drift.)
Thing is, it keeps him focused. Moving. Follow orders, like a train on a track. Survival: that's all instinct. When bombs are whistling, when machine guns are firing, he has to think *fast*, no time to worry or doubt....or drift. Here, in the war, you act.
(Except for all the time you spend waiting, bored and scared and uncomfortable and lonely. Waiting's worse than fighting, except for how it really, really isn't.)
Thing is, at home...
At home, safe, working in the recruitment office, getting trotted out for the occasional military dog and pony show, it was all waiting. Drifting. He thought he ought to care--couldn't help himself, now and then: his commander’s lunacy, unfit young idiots wanting him to truss them up for the slaughter, Sarah. . . But even Sarah wouldn’t be enough to keep him moving, feeling, not for days and years.
At home he didn't sleep. When he started to drift, it'd be bodies and blood and screams, his hands doing unimaginable things, not just to survive, but for no damned reason. Awake all night, drifting through the days like a sleepwalker.
Here, in the trenches, he sleeps fine. He's a soldier, he can sleep through anything.