First, it's important to understand that once you have the basics, happiness comes primarily from healthy social connections and a sense of purpose.
Many people with jobs have a fantasy about all the amazing things they would do if they didn't need to work. In reality, if they had the drive and commitment to do actually do those things, they wouldn't let a job get in the way. Unfortunately, if given a lot of money, they are much more likely to end up addicted to crack, or even worse
I used to watch a lot of cable TV (10-20 hours a week) and from time to time I would hit upon one of those celebrity biographies. As much as they tried to spin it, suddenly becoming rich and famous usually destroyed people's lives.
Meanwhile, some family friends we know who never made a lot of money retired, joined an RV traveling community. Every year the same bunch of people take several months touring the country. It is not very expensive. They are having the time of their life.
The point is this: Paul's advice is good even without the money. Social interactions (I would add travel) and a sense of purpose are foundational to happiness.
Somebody said once that money is a magnifier. It takes whatever character traits you already have and exaggerates them. I'm not an expert or anything, and I'd certainly like to be better off than I am, but I'm willing to bet that giving 5 or 10 million bucks to most people would be about the same in the long run as shooting them in the head.
Happiness, by its very nature and definition, is rooted in feeling pleasure about the happenings and circumstances of life. It's fickle and never guaranteed.
Joy and peace (a positive confidence despite the sad or happy circumstances) comes from having a set of principles one adheres to and complete self acceptance.
Paul's advice here is utterly incorrect and sounds ridiculous to even read:
...if you quit your job and move to a new city where you don't know anyone or have a clear purpose, there's a good chance that you'll end up depressed or even suicidal.
Seriously? People should spend less time seeking happiness (a fickle emotion), and spend more time becoming at peace with themselves, because inner peace will carry you through no matter the circumstances.