Can it be said that defective thinking, wrong perspectives and ‘set’ together account for all bad relationships?

Not for all, that would be too sweeping a statement, but they account for about 95%. One unfortunate aspect is that there is a tendency with some personalities, to project on to one with whom they have a relationship characteristics which are actually their own. For instance, a person may begin to think that his or her partner is acting strangely, or is over-interested in this or that, or not interested enough in something else, when all the time it is not the partner who has changed but the person’s perspectives which have undergone a subtle transformation, so that he or she views the other differently. Once a distorted viewpoint becomes set then the chances are that that the relationship will become strained. People rarely fall out of love with someone because of changes in that someone, but because of changes within themselves.