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
Well, I wouldn't assume that
If we used a trading analogy here, in the past we could only make long positions and prepare ourselves for the crash. In today's world we can short almost anything, and thus we should also prepare ourselves to the crash in reverse. In other words, being permaoptimist nowadays is as a losing strategy as it has become for being a permapessimist
We should be realists, but definitely not along the commonsensical view as some middle ground in between, i.e. neither here nor there. Realistically, we should be optimists when if serves us right and pessimists when it serves us even better. And that means in-depth understanding of and insight into whatever we are engaged or involved in