Our brains crave coherence. They want our lives to make sense. To do this they make up stories. We then cling to those stories and make ourselves happy or unhappy as a result.
If life doesn't fit our story we get frustrated and upset. If it does fit our story we feel pleased with ourselves. But then we worry that it won't last and things will change for the worse!
In reality our thoughts, even about exactly the same situation, change all the time, moment to moment. If we didn't add the thought of our made up story, of "the way things should be", we would be so much more at ease with ourselves -and life.