One of the things we look for when we're trying to understand how your defense mechanisms work is your ability to see the good and bad in something (or someone) and hold onto those attributes at the same time. To be able to see wholeness as a integration of good and bad into a cohesive sort of grey, rather than splitting everything into good or bad and going through enormous effort to put those two categories into some sort of tension with each other, and then in that tension the good and the bad can never exist at the same time.
When you're with someone that loves you one minute and hates you the next, its an expression of that kind of thinking. The idea being that we all have personality structures, and depending on the structure, how one organizes the sense of self and sense of other is an indicator or whether or not we're confronting neurosis or a deeper defict like a personality disorder. *Except in children, where this kind of thinking, the "friend/enemy list", is a normal part of their development*
The main problem or side-effect of black and white thinking is that in order to hold onto the division between good and bad, or render everything into those categories, one has to distort reality. And those distortions cause all kinds of problems (as a metaphor, think of a black and white photo instead as a black or white photo and you can see how the absense of the grey scale eliminates information). Or worse, when reality forces good and bad together and one starts to lose a sense of integrated-self (which is the fuel for acting out). That acting-out of emotions is also normal for children, but not a functional coping mechanism for an adult, rather it's a method of quick disposal of difficult feelings and often involves some sort of conflict or drama and suggests an injury or traits of some kind of personality disorder.
So, there's that.