I have been hesitant to bring up Borderline Personality Disorder, as I have associated with many of them in my lifetime. It is not a pretty picture.
Where to start? First of all, it is women who mostly have this problem. The psychology professionals say that it comes from infancy, when their needs are not met; they cry and they are not attended to. They come to believe that they can not count on others; or rather this thought is always in their mind, even if they are not thinking that right now.
With respect to other people, they move from idealization to complete contempt, just like that. From “I love you forever, it is great all that you have done for me” to “I hate you, I never loved you, how could you have been so mean to me?” It is like a switch, all or nothing. What is happening is that the Borderline believes that they have found the perfect other that can take of their needs, a desire so very strong after their problem infancy, then something happens to trigger the thoughts of can’t trust anyone that is never really gone, just temporarily hidden. Also note, that since women are programmed to seek out resources to satisfy their needs, this could be a rather important issue for them.
For the other person, it is like being hit by a semi-truck. It seems to come totally out of the blue; it is totally irrational. But for the Borderline, it is all very real, which means problems for them and everybody around them