One aspect of the setting that is used to reveal the character of Jimmy Cross is the way that these soldiers are constantly surrounded by danger as they "hump" their various belongings up and down hills and complete their various missions. Yet the danger that these soldiers face often turns into merely another opportunity to reflect on their various obsessions and fantasies that help get them through each day of their terrifying existence. Consider the way that this is shown through Jimmy Cross's response to finding a tunnel:
After five minutes, Lieutenant Jimmy Cross moved to the tunnel, leaned down, and examined the darkness. Trouble, he thought--a cave-in maybe. And then suddenly, without willing it, he was thinking about Martha. The stresses and fractures, the quick collapse, the two of them buried alive under all that weight. Dense, crushing love.
Note how his private fantasies of a woman who he is not even dating intrude into his job as Lieutenant (interestingly, he is always refered to in the story as "Lieutenant Jimmy Cross" as if to emphasise his position). Jimmy's response to letting this flight of fantasy result, as he thinks, in the death of Lavender, is to burn his letters to Martha and to end his fantasies, focusing only on his job and looking after the soldiers entrusted to him. Note the resolutions that he makes:
He would not tolerate laxity. He would show strength, distancing himself.
Lieutenant Jimmy Cross therefore resolves to learn his lesson from Lavender's death, no longer letting himself focus on what is not real to the distraction of the unyielding and harsh reality of his situation. His new hardness is expressed through his intention to "distance himself" from his men and the way that "laxity" will no longer be permissible.