Desire is what your hero wants. Just as every sentence requires a subject and a predicate, a story should, if nothing else, feature a hero wanting something. Kurt Vonnegut said, "Every character should want something, even if it is only a glass of water." If there is nothing that your hero wants, then there is no story, no journey from Point A to Point B. If I want a glass of water, the journey involves me getting up, going to my kitchen, finding a cup, and pouring the water. That's a story. A lame story, but a story nonetheless.
What is Joker's desire in Full Metal Jacket? I think in both parts, he really wants to survive. Clearly, once he's in Vietnam, he wants to get out of the war alive, but even at Parris Island, once he realizes how brutal Hartman can be, Joker wants to get out with his sanity intact.
The idea of mental survival in addition to physical survive is an interesting one to me, and I think it's more important in a good story than just the physical side. If you look at any story involving survival, you'd be hard pressed to find one in which the hero emerged without some psychological scar. That's probably because survival has to be a tough, harrowing experience. It's a struggle between life and death. If a character gets lost in the woods for a week, and comes out to return to business as usual, then there wasn't that much of a struggle, was there?
If you want a good example of mental survival, you might want to check out the Tom Hanks film Castaway.