The question of who lives and who dies in Night stems from a system of unnatural rules. SS officers, such as Dr. Mengele, play God and decide who will live or who will go to the crematorium. "Selection" is based on perceived ability to work, so the young, old, and sickly are quickly killed, as are those who fail to work hard enough. Because the system determining life or death is subjective, some prisoners are able to play the system and can avoid death (to some extent) based on cunning. For example, Eliezer lies and says that he’s 18-years-old and a farmer, which probably saves him from the crematorium when he first arrives at Birkenau. But despite this system of selection in Night, death and survival are often also based on chance. Using the same example from earlier, if another prisoner had not randomly advised Eliezer to pretend to be 18-years-old, he may have been sent to the crematorium immediately upon arrival in Birkenau.
Although death is omnipresent, it does not become real to Eliezer until his father dies.
Eliezer is confronted with his mortality and realizes the likelihood of his own death on his first day at Birkenau when he sees children burned alive.