The ruminative state of mind is actually a variant of a much more general mode of mind that has been called the “doing” mode. The job of this mode of mind is to get things done—to achieve particular goals that the mind has set.
These goals could relate to the external world—to make a meal, build a house, or travel to the moon—or to the internal world of self—to feel happy, not make mistakes, never be depressed again, or be a good person. The basic strategy to achieve such goals involves something we call the “discrepancy monitor”: a process that continually monitors and evaluates our current situation against a model or standard—an idea of what is desired, required, expected, or feared. Once this discrepancy monitor is switched on, it will find mismatches between how things are and how we think they should be.
That is its job.
Registering these mismatches motivates further attempts to reduce these discrepancies. But, crucially, dwelling on how things are not as we want them to be can, naturally enough, create further negative moods.
In this way, our attempts to solve a “problem” by endlessly thinking about it can keep us locked into the state of mind from which we are doing our best to escape.
100 Words Everyday 