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.