Your brain can’t process billions of pieces from sensory inputs.
“Reticular Activating System” is a filter that selectively allows certain information into the brain.
This filter is programmed by your thoughts and beliefs (regardless of your past experiences).
It allows the pieces of information that you like to see/feel or what you believe, only to strengthen it.
You’ll get what you expect to get. Everything this filter allows only helps you to add up what you already have.
Being optimistic is scientifically proven as good strategy.