If Thomas Sr. would have get out of the depression, he could live that second chance he got for living but he neglected it. In other words, this depression has psychologically and physically weakened and destroyed him. .
The hatred and depression of Oskar's daily life led him to changing his behavior. Oskar used to spend most of the time with his dad and when he heard the news of a plane hitting twin towers where his father was, he thought his father would comeback but he didn't. This made Oskar depressed as he thinks this should have never happened because he loved his dad a lot. Oskar starts living his life alone and quietly and distanced from his mother. He rarely talks to her mother and started feeling lonely and afraid. He starts to hate the world and everything around him. He also had a mission to get the lock to the key which he thought would make him closer to his Dad because he found the key in his Dad's closet. The author wanted the readers to realize the effects of depression on a person's life. He wanted his Dad to comeback so when his mom was talking to him about his dad he freaked out and said, "If I have chosen you, I would have chosen you" (Jonathan Safran Foer 171) which means that his attitude was changing after his father's death and due to depression and he is so upset that he want his dad to comeback even though he has to give away his mother. Another attitude changing moment was when his mom asked if he is an optimist and he gave an example of being an optimistic which was "Why do beautiful songs make you sad" "Because they aren't true. "Never" Nothing is Beautiful and true" (Jonathan Safran Foer 43) which according to the author does not sound optimistic. This quote also shows the hate he has been developing inside due to depression. .
The depression and hate affected Oskar's life as he faced challenges to get through depression.