The only hint of a suicide note left by Charlie Hartz is that he kills himself on Paradise Lane. Jeremy is convinced that this is "corny" and an "existential gesture." The narrator's father tells her that Paradise Lane is meaningless.
Charlie has a wife, a child, and lives in an extravagant house with a huge swimming pool. He is a successful businessman. He has friends and is always interested in being productive. He enjoys diving and is "very determined about it." He is forthright when talking the narrator and younger people and they respect him for it. Charlie even provides opportunities for the narrator and Jeremy to be together. The narrator also says that Charlie was interested in the lives of her sister, Jeremy, and herself:
What kept people like me and Jeremy and my sister around him was that Charlie got the best out of what you were at the time you were it.
Other than the narrator's comment that Charlie rarely laughed, there are no real indications that Charlie is depressed or upset. No one really knows why Charlie killed himself and this is the point. Charlie seemed to be a stable, relatively happy person. He was always involved with some project with a number of people in his life. He took an avid interest in other people and encouraged them to better themselves. Perhaps one of the points the author is trying to make is that every person has a story. Every person has their own battles to fight and their own emotional and mental problems to face. Some, such as Charlie, do this internally. Evidently, no one had gotten to know Charlie the way he had gotten to know everyone else. As much as he reached out to others, someone should have reached out to him.