Here's an article I liked about how Black and White thinking can promote your survival under certain circumstances, but can also lead to depression, if you do too much of it.

All or Nothing', or 'Black and White' Thinking and Depression

I've known people who do B or W thinking excessively. When they talk about someone, the person is either a saint or that person is the devil. They can't seem to accept that most people are a little of both. They think someone is their savior, or their mortal enemy. So they imagine that a person is such a good friend that they would die for them, or else a person is their enemy out to destroy their life. They think in extremes.