How do characters change or evolve throughout the course of the story? What events trigger such changes?

For this question I will focus on Holden Caufield character for this question. Holden's character is quite interesting and confusing. Much like his emotions, Holden's personality is very scattered, which makes it incredibly difficult to observe any change or growth within him.
 However, what could be discussed in this question is Holden's personality prior to the time frame of the novel. As many people who have read the book know, Holden had a younger brother named Allie who sadly died at a very young age. Upon learning of his brothers death, a young Holden went into a rage mode and proceeded to smash the garage's windows out with his bare hands. He never wanted go for help to accept his brother's death as he would say that he's okay.
This one moment changed his life completely, as he changes from an unassuming normal and kind boy into a depressed jerk who goes around hating everyone and everything in the world. Holden's one sided brawl is a top example of his character going through an enormous change.