It is typical for patients with borderline personality disorder to idealize another person as being "perfect" or the "best person I ever met". Due to dichotomous thinking and splitting, people with this type of personality tend to see only extremes, like "all good" or completely bad. They are not able to realize that someone will not be able to meet all their needs or be present every minute of the day and night only for this one person, but would be willing and able to meet some of these needs and accept the person as a valuable individual. So the borderline patient tends to avoid unpleasant (realistic) aspects, feelings or experiences. Their view can rapidly shift from positive to negative and from one view to another, without being able to combine the shifting views into a complete whole. This judgement can be rather intuitive and is not based on facts or recent experiences. Very often this is more or less influenced by old schemes or dreams of the past.

If something interferes seriously with this idealization of a "beloved person", very extreme behaviours may result. One possibility is the complete devaluation of the person. Now all personal qualities and behaviours of the other person are judged to be awful, harmful or are seen as a tricky way to manipulate or abuse oneself. Again, this is mainly based on old (bad) experiences, not the current situations or behaviour of the people involved.