Why is it we'll stew for hours about the person who drove badly in front of us, to the exclusion of noticing what a nice day or otherwise uneventful drive we had?
Because we're supposed to notice the bad things and learn from them, and not notice the things that go well. As neuropsychologist Rick Hanson puts it, the brain is Velcro for the negative, Teflon for the positive. He explains why--and how we can work around that architecture--in his book Hardwiring Happiness: The New Brain Science of Contentment, Calm, and Confidence.