The simple explanation is that bad events are more important for our survival, while the good ones not so much. If we don't learn our lessons (read, quickly forget about troubling experiences), we risk getting into the same situation again and again. Put differently, the natural selection has efficiently got rid of those enjoying excessive amounts of optimism. So, in a sense, we are all innate pessimists
Now the mucky business of actual survival is largely solved we need to start evolving the other way otherwise pessimism will kill us with hopelessness and division.
I've gradually stopped caring about anything beyond my control and this year has certainly helped accelerate that. Cutting the media out of your life entirely probably does more for your well being than running five miles a day.