I think that the previous post was very strong. I would only add that part of what moved Eliezer closer to a feeling of despair and hopelessness was the wounding of his faith and renunciation of God as a result of his experiences. Part of what makes Wiesel's work so powerful is that it displays the horrors of the Holocaust, but also identifies how the death of one's faith can prove to be an irreparable blow to one's courage and ability to persevere. In chapter 4, when Eliezer sees the smokestacks of Auschwitz, the incineration of babies and children, and the lack of verifiable answers as to why such atrocities happen and where God's presence is in such a condition, this marks a critical moment when Eliezer moves increasingly closer to a realm where agony becomes the dominant mode of existence.
Most of his frustration and depression has to do with how he witnessed the unraveling of human instinct in a moment of extreme circumstance.
For example, when on the cart, he sees the man and the woman exchanging vulgar sexual messages as the fear of dying was rampant in the small space. This is convective because it shows how he, as a child at the time, saw the dark side of desperation and insanity.
This is also evident through hunger. People's hunger was so intense that it changed them into survivors. His father, for example, nearly shifted his role because he was starving for more soup and bread, and both parent and son did not want to take it from each other.
The hundreds of bad news, the people dying, the fear of being burned alive, the ovens- the entire scenario was too chaotic for a man so young. Therefore, his only choice was to try his best to not lose his sanity.