One of the primary reasons that Eliezer survives is that his experience slowly withers away his humanity, making survival almost animalistic. Throughout the course of the narrative, Eliezer slowly loses those bonds that help to define his humanity. His community, his religion, his faith in human goodness, his mother and sister, and finally, his father. The severing of these bonds through his experiences at the different camp allow him to become more primal in approaching survival. It gets to a point where he wishes his father no longer remain with him so that he can simply live. One might call it a will to live, a fight for survival. Yet, as with much that Wiesel depicts, I think that it might be more of a statement on the dehumanization of the Holocaust. Animals fight to merely survive, while humans live life. The experience of Holocaust victims dehumanized them to a point where survival was all that mattered, making them resemble something other than human in the end. This might be where Eliezer was in terms of how he was able to survive his experiences. Consider this assessment of Eliezer's character:
Consequently, he [Eliezer] is bent solely on survival, and only his stomach takes note of time. Still he survives but merely as a corpse in a mirrored gaze just waking up from the long night.