Lt. Jimmy Cross is an idealistic man. His draught to war signifies more to him than just a task: He believes that he is somehow selected to perform a duty that is almost otherworldly.
When he becomes "a soldier", Jimmy takes on the role with somewhat of a knightly disposition. For example, he idealizes Martha as some sort of damsel in distress who yearns for him and writes him letters of love.
He imagined bare feet. Martha was a poet, with the poet's sensibilities, and her feet would be brown and bare the toenails unpainted, the eyes chilly and somber like the ocean in March, and though it was painful, he wondered who had been with her that afternoon. He imagined a pair of shadows moving along the strip of sand where things came together but also separated. It was phantom jealousy, he knew, but he couldn't help himself. He loved her so much.
He also feels that he is responsible for the death of Lavender, for the choices that other soldiers make, and for the outcomes of his platoon. Surely he is partially responsible for his men, but only in the capacity of a military officer, not because he has any kind of divine assignment in the lives of the other men.
However, the point that O'Brien wants to make in "The Things They Carried" is that Jimmy's attitude is not uncommon for many men to assume during circumstances like that. A man who suddenly becomes a soldier is given responsibilities and tasks that he has never performed before. Additionally, he is made aware of his important role in the lives of men that are as scared and shocked as he is.
The fact that Jimmy is more educated seems to allow him to make the experience all the more theatrical in his mind. Even if men know things for what they really are, they are prone to make much more of it in a stressful situation. Although Jimmy carries Martha's letters around and uses them as a devotional object, the facts still remained:
The letters weighed ten ounces. They were signed "Love, Martha," but Lieutenant Cross understood that Love was only a way of signing and did not mean what he sometimes pretended it meant.
In the end, Jimmy realizes the importance of valuing reality. As death gets closer and closer to the platoon, Jimmy slowly understands that life has to be taken for what it is. No gimmicks, no sad stories: He has a task that he must complete. Hence, the way he changes is by finally accepting that reality may not be as fun as fantasy but, in the end, holding strongly to it will be what ultimate saves their lives.