Let’s assume for the moment that your expectations affect reality.
There doesn’t have to be any weird supernatural basis for that. You affect reality with your mind every day. There are a lot of things in the world which are out of your control, but your actions are always under your control, and they certainly affect the rest of the world. And your actions are a direct consequence of your thoughts.
That wasn’t so hard, right? A very simple, direct line between your thoughts and consequences in the real world.
You can’t win the lottery by thinking positive thoughts (as far as I know). But you can notice opportunities and act on them. You can believe in yourself and take chances which might not work out. You can trust your intuition and use it to keep you pointed in the right direction.
None of it can happen if you’re pessimistic, inflexible and locked into a fixed mindset. Optimism is something you can practice, and it’s totally worth it.