When people get too stressed they don't slow down to think. And tend to do and say things without really thinking, which usually leads to an increase in the mistakes they make. Therefore leading to more stress, which leads to even less clear thinking. When this cycle continues over long periods of time(months, years) people begin to believe that's how they should be thinking. And the quality of their lives sometimes begins to slowly spiral downwards in many ways. They know some things are getting worse for them, but they sadly don't realize it's because of the way they are thinking. They want to believe their logic is correct since they've been getting through life using their logic at that point.

 This is why it's important to not get too upset over things that barely matter or don't matter. Because you could be using that focus and energy on things that are actually important to you.