Therapy and holding "Two truths at once”

As people, we tend to seek out black and white thinking . Some examples- if a person is in this group, they are not in THAT group. If this person believes this, they are bad. If they believe that, they are good. If I feel a certain way, all other evidence to the contrary must be wrong. Of coures, the reality is often so much messier, but also so much more expansive and adaptive. So, Why do we do this?