Charlie Hartz is quite a mysterious character in "The Man Who Jumped Into the Water." As a result of this, your question will always be hard to answer. We are only give a tiny thought about the suicide (simply that it happens) and given nothing about the "why" you ask for in your question. However, if we take a closer look at Charlie's life, we can take an educated guess as to why he took his own life. In my opinion, the reason is something that is often common with very rich people: money does not buy happiness.
First, let's look at Charlie's life. Charlie Hartz is a rich man. He seems to have everything a person could want. Charlie has a nice family: a wife and child. Charlie has a beautiful and extravagant home with a swimming pool. Charlie has an athletic streak in him: he loves the sport of diving. Charlie has many friends, including the narrator. In fact the narrator says the following about the attraction towards people like Charlie:
Charlie got the best out of what you were at the time you were it.
Charlie is also successful in business (which allowed him to make all of his money). As a result of all of these things, Charlie is the definition of American success.
Now let's talk about the real clues from the text as to why Charlie might take his own life. There are two real clues (albeit indirect). The first is that narrator's comment that Charlie "rarely laughed." What does this show? Charlie, for one reason or another, was unhappy. Laughter is conducive to happiness and exemplifies the presence of joy. Despite Charlie's efforts to better humanity, we can assume that Charlie must have been bereft of joy. How sad for him. Further, we get a clue from our narrator's dad who says that Charlie's beloved Paradise Lane is "meaningless." If any part of a rich man's life is "meaningless" and the rich man doesn't laugh at all, we can assume he is not happy. This is the best guess a reader can make as to why Charlie killed himself.
In conclusion, one must recognize the similarity of Charlie to other famous characters in American Literature such as Richard Cory (from the poem of the same name) and Jay Gatsby (again, from the book The Great Gatsby). Richard Cory shoots himself in the head and is an almost exact replica of Charlie. Jay Gatsby is shot by a Mr. Wilson, but is also an unhappy person. The ultimate conclusion: money doesn't buy happiness.