The more intelligent you are, the higher probability of not being happy. Happier people tend to not be deep thinkers, tend to not dwell on the future or the past. They live in the moment experiencing joy of the present.
I suppose those lucky non-ambitious and rather frankly either lobotomized or only seeming so people may be happy. But even those people living traditional tribal lives who may seem happy from their subsistence farming have their own ambitions, worries and concerns for their lives and those of others. The book was alarmingly fatuious.
I don't believe in destiny. I believe in patterns and free will. We have the individual choice either given by grace of a higher being or by environmental nature (what you will) to either recognize the patterns of life or to boldly ignore them as creatively as possible. What I don't accept is the notion that failure is a viable option, that ignorance is acceptable as an alternative to thorough thought, and that the current time is 2:30am...wait, back up - scratch the last one.
I suppose that with everything, it is a moderation of these feelings - frustration, anticipation, fear, perfectionism some semblance of joy can be found. I love my family and my friends, however I truly believe that I am somehow, without realising it, distancing myself from them.