Pessimism as hubris
It seems to me that pessimism in the face of major problems is sometimes an act of hubris. I sometimes find myself pessimistic simply because I don’t see any way of solving major problems such as securing the world’s nuclear materials (still a surprisingly scary problem), or solving the climate change problem. But, of course, just because I don’t personally see how to solve a problem doesn’t mean a solution won’t come from a direction I don’t expect, and perhaps have never even heard of. I remember many discussions of nuclear war in my school in the early 1980s, and the mood was always very dark; no-one anticipated that the breakup of the whole eastern bloc was just a few years away.
Taken too far, this point of view can lead to foolish optimism: “someone will save us!” The ideal frame of mind seems to be to balance optimism, on the grounds that there is far more ingenuity in the world than we are aware of, with a certain amount of pessimism, to remain strongly motivated to act ourselves. The sky really has fallen on some civilizations.