I believe feeling good about the future with many specific reasons to feel good about the future is totally valid though.
Ask yourself why people in the slums of Tanzania might not be "optimistic about the future". Is it because of their poor current living situation, or is it because they don't see as much of the political, scientific, and social progress being made over time globally? I'd argue it's more likely the latter.
I don't know what's going to happen in the future. No one does. I don't have all the facts. And even if I did, I would need a galaxy brain to integrate them all into an outlook. Not everything is publicized. There are many groups of people who work to further their own agendas. We aren't one globe, one humanity. It's a nice idea, but it doesn't reflect reality.
I base my pessimism on the childishness I see flourishing in adults who have lost contact with reality because they've never endured serious hardship. These people can be easily manipulated because they have little fear. They have never made life or death decisions. So they don't understand the significance of forming an accurate model of the world. The result is a nation full of dreamers. Such people are ill adapted to the real world and will naturally be ineffective in all that they do.
There are very strong incentives for forming an optimistic outlook. None of these are conditioned on data or serious consideration. I think people look for reasons to justify how they want to feel. In other words, optimism precedes the reasons one has for being optimistic.
Voltaire lampooned optimism very well in Candide. His ultimate conclusion was: don't waste your time on idealistic notions of a world in which terrible things are always happening; instead, tend to your garden--do your best to improve what you can and no more. This is a wise suggestion. Optimism and pessimism are both traps. They are pathological extensions of present conditions.
Ask yourself why people in the slums of Tanzania might not be "optimistic about the future". Is it because of their poor current living situation, or is it because they don't see as much of the political, scientific, and social progress being made over time globally? I'd argue it's more likely the latter.