In general, the Dutch are happy, the Romanians aren't, and we here in the good ole US are somewhere in between. Thailand, in fact, has a Gross Domestic Happiness Index that the government established to ensure fun is had by all. On the other end of the happy spectrum are the wealthy Qatar citizens who lead unhappy lives in a "gilded sandbox."
Weiner becomes involved with the citizens of these happy/not-so-happy countries. He eats rotten shark meat in Iceland, visits strip clubs in Bangkok, and smokes Moroccan hashish in Rotterdam, all in hopes of making the reader happier.
"With our words, we subconsciously conflate geography and happiness. We speak of searching for happiness, of finding contentment, as if these were locations in an atlas, actual places that we could visit if only we had the proper map and the right navigational skills. Anyone who has taken a vacation to, say, some Caribbean island and had flash through their mind the uninvited thought 'I could be happy here' knows what I mean."
So, where is the happiest place in the world? You'll just have to read the book to find out.