What drives Louis's determination to survive the grueling conditions of both the open water of the pacific ocean and the POW camps?
I'm looking for the cause or motivation for Louis's will to survive.
I think it could be argued that Zamperini's motivation to live was not driven by an external factor (say, a relationship back home) but rather, came directly from within. It was in his nature to be the very best, to live life to the fullest, to fight, and to win. In his experience as a POW, this natural sense of personal ambition manifested itself in the will to survive.
Consider his character even as a boy. First, he is rebellious in his youth and prided himself on escaping the punishment of the law. Later, he channels this competitive energy into running, and makes up his mind that he will become the best runner in the world. After succeeding as a high school track star, he makes it to the Olympics then goes on to break several track records as a college athlete.
In many ways, his natural physical talents are the embodiment of what must certainly be an even stronger mental capacity. It is as if Zamperini had already tested the full limits of his body before becoming a POW. This, in turn, gives him the advantage of knowing exactly how strong he really is (both mentally and physically). Though it is never explicitly stated, certainly, this hero's will to survive was something he possessed long before he became a prisoner of war.
Even after his liberation, his goal continues to be to "live life to the fullest." After a battle with depression and alcoholism, at the end of the book it is revealed that Zamperini becomes a born-again Christian as a result of a Billy Graham crusade. It is then that his personal war ends (the depression, the flashbacks, the alcohol: gone). Thus, it could be further argued that he not only possessed a strength of his mental character, but perhaps, his spiritual self as well. This detail poses the suggestion that it is by a force of God that Zamperini was meant to live all along.