In The Things They Carried, why does the lieutenant burn the letters he has been carrying?

In the first chapter, O'Brien recalls how Lieutenant Jimmy Cross would carry around letters from Martha, a woman he loved, and describes how Jimmy would daydream about her while he was out in the jungle leading his troops. One day, Lieutenant Jimmy Cross and his men are clearing a tunnel when he begins to daydream about Martha. He cannot help himself and becomes utterly lost in his thoughts. Suddenly, Ted Lavender gets shot in the...

In the first chapter, O'Brien recalls how Lieutenant Jimmy Cross would carry around letters from Martha, a woman he loved, and describes how Jimmy would daydream about her while he was out in the jungle leading his troops. One day, Lieutenant Jimmy Cross and his men are clearing a tunnel when he begins to daydream about Martha. He cannot help himself and becomes utterly lost in his thoughts. Suddenly, Ted Lavender gets shot in the head on his way back from taking a pee. Lieutenant Cross feels guilty and responsible for Ted Lavender's death and ends up burning Martha's letters and photographs at the bottom of his foxhole. Lieutenant Cross feels that Lavender's death was due to his negligence and believes that thinking about Martha keeps him from staying focused on the safety of his men. He is also upset at the fact that Martha does not love him back. Burning her letters and photographs is Lieutenant Cross's gesture of attempting to forget about Martha and focusing on his job.