Hope
(He was a priest. He wasn't Jewish--but very intelligent!)
Art Spiegelman, Maus II, p. 28.
While luck may have given people they opportunities to survive. They had to hold on to something so that they would live long enough to get those opportunities. In one respect Valdek and Anja clung to their love for each other. Knowing the other was alive gave them a reason to live. This idea is reflected in the website Auschwitz Alphabet (linked above) under H - for hope. Hope of a better what is what sustained many people during the war.
The above quote demonstrates another kind of hope, belief in religion. When Valdek is truly beginning to despair, a priest comes to him and shows him that the number he has been branded with totals the Hebrew number of life. For Valdek, this is a fresh hope, something new to cling to in times of despair. Beliefs such as these were especially important, but especially hard to hold on to. Another example is that Valdek had a dream on night of his late grandfather telling him that he would be freed on the day of Parshas Truma. When he tells another prisoner, the reply is, "Let's hope it's true. I'm afraid we'll never get out of here." (I.57) Visions such as these gave the prisoners some hope that their struggle would not be in vain.
However, in the face of such utter despair and destruction, it was difficult to believe in a God. In the words of Elie Weisel, contained on the same site as the above quote from MAUS, "Never should I forget that nocturnal silence which deprived me, for all eternity, of the desire to live. Never shall I forget those moments which murdered my God and my soul and turned my dreams to dust." Nonetheless, even through the war murdered his God, Weisel foun something to sustain him throughout the war. whether it was the belief in a better life, belief in God, or the love of a spouse did not matter. It was a kind of hope and that was what counted.