There are two major events in Holden's past related to death: his brother Allie dying from leukemia, and James Castles's suicide at the Elkton Hills school. How has each of these events affected Holden and his thoughts about mortality?
One way to answer this question is to cross reference chapter 14 with chapter 22 in Salinger's The Catcher in the Rye. In chapter 14, the reader gets a glimpse into how regretful Holden feels about a decision he made in the past with his brother Allie; that is, he told Allie to go home rather than allowing him to go with him and a friend. Holden says that he talks to Allie when he is depressed and tells him to come along with him. The connection here is that Holden does acknowledge that Allie isn't coming back and that death is real. However, when he is talking with his sister in chapter 22, he speaks about Allie as if he were still present and Phoebe feels the need to remind him that their brother is dead. It's as if Holden can't consistently accept Allie's death. When Phoebe asks Holden (chapter 22) if there is one thing he likes in life, he answers with "Allie" but then thinks of another death in his life when Phoebe rejects that answer as mentioned above.
James Castle committed suicide after being bullied by some boys at Elkton Hills school. Holden's thoughts must have drifted from Allie, whom he loved, to the only other death he had experienced up to that point in his life. As a teenager, both experiences with death were very traumatic and involved young people who should have lived out long lives. Teens usually feel invincible but Holden witnessed death very early on in life. He even describes James Castle's teeth having been strewn around him on the steps where he was found (170).
Because of these experiences, Holden must view death as an escape from a dark and tough world. Later, he even says that if he were to go to war, he'd rather that they just go and shoot him rather than make him suffer through living in the army for so long. He references this remark to his older brother's time in the army and seems to imply that living just isn't as easy as dying. Even throughout the book Holden complains about almost everyone and everything. He hates living, he sees no real purpose in life, and can't come to a decision about whether to live life (in reality) or simply try to avoid it.