Humans have to assess all incoming information (gossip round the campfire, stirring in the long grass or twitter feeds) and aa strong bias to pay more attention to potential negatives in the environment will in general allow us to avoid disasters (like getting eaten) In our evolutionary environments that would be balanced out by "good fortune" - a old tree has just produced fruit perhaps, so we would get our optimism…
"In short what used to be good news (hey I got to eat today) is just normality and so that leaves just downsides (perhaps an analogy of a roller coaster with half of it underwater - being above water is taken for granted and so there is only dips down and long slow climbs back to normal.)." I think it comes down to income inequality. A lot of people don't share in benefits of a rising economy so they become cynical a…
now I would suggest our pessimism gets much more work out
However an interesting podcast (frwkonnomics) has someone last week saying their research showed that we are more pessimistic when stressed and more optimistic when less stressed (indicating that perhaps it is just the stress of the information overload leading to pessimism rather than the information content)