I read that book coming to the conclusion getting rich is not worth it and subject to luck and a lack of mental health. Which I think is pretty much what Felix Dennis intended.

I bought the book expecting practical advice (I was young and had no love for myself), but was surprised by a rather poignant read. It's been a long time, though. I remember mostly the part about hookers and drugs ruining his health.

I think it technically only implies that at the time when he first became rich, he didn't understand why. It's possible that he has since discovered what exactly happened.

As a sibling comment already implied, the core message of the book is that the price of getting rich is often not worth it. I personally also enjoy the "Rich but not famous" essay by Tim Ferriss at https://tim.blog/2020/02/02/reasons-to-not-become-famous/ which has a lot of vivid descriptions of the downsides of fame.