Rick Hanson, author of the recently published Hardwiring Happiness: The New Brain Science of Contentment, Calm, and Confidence, spoke Wednesday to The Atlantic about why our evolutionary wiring can make it harder to ‘build’ happiness:
There was a lot about [hunter-gatherer life] that was very hard: there was no pain control, there was no refrigeration, there was no rule of law. Childbirth was a dangerous experience for many people. There’s a lot about modernity that’s good for the Stone Age brain. We do have the ability in the developed world—far from perfect, of course—to control pain. We have modern medicine, sanitation, flushed toilets and so forth and, in many places, the rule of law. …
… And yet on the other hand, many people today would report that they have a fundamental sense of feeling stressed and pressured and disconnected from other people, longing for closeness that they don’t have, frustrated, driven, etc. Why is that? I think one reason is that we’re simply wasting the positive experiences that we’re having, in part due to modernity, because we’re not taking into account that design bug in the Stone Age brain that it doesn’t learn very well.
This “design bug” is also our wired tendency to focus on the negative (got to be wary of tigers). Hanson’s solution? Making sure core needs — safety, satisfaction, and connection — are met and repeatedly internalizing these, taking “the extra 10, 20, 30 seconds to enable everyday [positive] experiences to convert to neural structure.”
You can read excerpts of the book on Hanson’s site.
Weight Maven is written by Beth Mazur. Beth believes that obesity is more symptom than cause and that the real problem is our Western diet -- especially sugar, refined grains, and industrial oils. Beth writes about nutrition, ancestral health, & food policy. And cats! 
Another theory that I heard recently is that even though we still have the flight-or-fight response, we don’t let go of fear or worry once the “threat” is gone; the feeling stays with us.