Sometimes it's not optimism that is important but a little dose of pessimism
A lot of insight into how we should approach life is often given and needed. And sometimes it's best to be hopeful even in the worst-case scenarios, and if you're not, at least try your hardest to achieve some of your dreams.
Source
By being pessimistic and expecting the worst, we just end up disappointed in the end. Hope for the best while expecting the worst is also a phrase used as a positive mantra. It helps us stay focused on what we can control instead of getting overwhelmed by what we cannot control and letting failures get us down.
This is why you have to expect the worst, so you'll appreciate the best. when it comes to coming.