The basic thesis is that we predict our future happiness by using our imagination to generate an idea of the future and then responding emotionally to that imagined future.
The process suffers from a number of cognitive distortions. For instance, we are overly biased by our common emotional state. And even if we are aware of the bias we never learn to correct for it (similar to how even if we are aware of an optical illusion we still "see" it).
Gilbert than suggests we are better off just asking people who are experiencing similar things how happy they are. A common objection is that we think we are a unique snowflake. But we're actually not
(1) The book is peppered with psych studies. I wonder how this holds up post-replication crisis revelation (2) Are we not special snowflakes? I enjoy SWEing well enough. I assume most people will hate it (3) Imagination is definitely easier than asking people