This article from the NY Mag Science of Us summarizes a nice selection of scientific research concerning what our brain likes to default to when it’s host is not engaged in doing something.
When you don’t give its human anything to do, brain areas related to processing emotions, recalling memory, and thinking about what’s to come become quietly active. These self-referential streams of thought are so pervasive that in a formative paper Marcus Raichle, a Washington University neurologist who helped found the field, declared it to be the “the default mode of brain function,” and the constellation of brain areas that carry it out are the default mode network, or DMN. Because when given nothing else to do, the brain defaults to thinking about the person it’s embedded in.
If you’ve ever experienced excessive rumination about the past or future or found yourself spiraling down a hole of thought for some other reason, odds are it’s because your mind is not engaged with doing anything else. It’s default mode of operation is to start thinking of all the ways it needs to plan and protect you and control your future by analyzing your past experience.