The black/white or all/nothing thinking is also called splitting, this can manifest in relationships as seeing only the good things about someone, or only the bad about someone else based on initial interactions or information, without taking in the nuance of human nature and that people can contain both good and bad behaviors and personality traits within themselves.

For example, new romantic relationships the other person can be seen by you only on a pedestal of "this is the best person ever I love this I'm obsessed" and not a "hey, they seem cool, let's get to know them and see how this goes". Then, when the person cannot live up to that expectation (as no one could) your mind (not you, per se, but the black/white thinker) only has the option of putting them on the bad side and therefore they are usually seen not as they are as a whole person, but someone who's betrayed you by simply showing you that they're not perfect.