... Why?

In chapter 20, Holden wanders around in a park and starts to worry about catching pneumonia. He starts thinking about his own funeral and then goes on to talking about his brother, Allie's funeral. Holden recalls his visits to Allie's grave and mentions that he didn't like going to the grave. He becomes disturbed at the fact that people place flowers at people's graves and questions why people would do such a thing for someone who is dead.

This "scene" doesn't necessarily confuse me but it makes me question Holden as a character. Why is he almost bitter about "putting flowers at people's graves" and why is he so disturbed by it? I'm just really confused at why Holden acts and thinks the way he does. I don't think he thinks rationally which causes him to not act sensibly.