This was certainly a better book than The Happiness Hypothesis: Finding Modern Truth in Ancient Wisdom, which sometimes had a feel-good-inc tone. I almost agree with all the ideas of Gilbert when it comes to happiness, though I didn't find all his experimental data decisive. Why I loved the book? I believe it makes very good sense. The beauty of it is that it doesn't need experiments to back it up. You know in your heart that what he is saying is true.
The main idea in this book is that we exaggerate how happy or sad we would feel when we contemplate certain scenarios about our futures. This happens because although we think we are simulating an event which lies in the future, we are not doing that at all. Our conception of future events cannot escape the influence of a) Our view of the world in the present moment b) Our memories of past event which are almost always distorted.
The book is well written, humorous (though sometimes he overdoes it) and interesting to read. It is not a self-help book, and sometimes even pessimistic about how we are bound to make the same mistakes again and again. There is an advise about how we can try to avoid making mistakes about our futures experiences which is pretty straightforward and convincing: we should seek the advise of those who are at the present living the futures we are visualizing for ourselves. However, this may not always be possible, especially if you are contemplating a future which is unique enough as not to find anyone to ask about it, not to mention those who aren't honest enough with you or themselves to share their experiences, which is sad but true. So, there no major advise here about how to become happy and hence the book's title which says it all. I can assure you that the happiest moments in my life were neither planned, nor expected. They were moments that I just stumbled upon.