A surefire way to destroy your relationship is to diagnose your partner—in your head, at least—with a personality disorder or other “character disease.”
Unfortunately, a cottage industry of self-help books exists to encourage you to do just that. Some are written by advocates or “survivors,” who describe how you should react to your partner, using incendiary and contemptuous adjectives—congenitally manipulative, opportunistic, cunning, exploitive, wolf in sheep's clothing, and the like. Others are written by therapists who psychoanalyze the partner with various interpretations of why he or she makes you feel bad. Both groups describe the insidious behavior of the personality disorder, not so much to inform and enlighten as to appeal to the reader’s victim-identity and self-righteousness. They typically supplement their superficial descriptions with general symptom checklists, without emphasizing that a preponderance of those traits—not just a few—must be present through adolescent developmental stages for a valid diagnosis and that a valid diagnosis can only come after careful, objective examination of the person being diagnosed.
The desire to hook clients into psychotherapy sometimes overrides the ethics of therapists who diagnose their clients’ partners, based entirely on third-party descriptions. (It hooks in clients because they long to explore what’s wrong with their partners.) I've had two dozen couples referred to me in the past year alone who were each diagnosed by their partner’s individual therapist with a personality disorder. The most common combination, of course, is a narcissistic man married to a borderline woman, though variations have come from more creative therapists with the temerity to diagnose without examination. In no case were these third-party diagnoses valid and certainly not helpful. Anyone who sees a distressed couple conjointly and the parties individually knows that neither are likely to present with a personality disorder individually but conjointly both do. It's the pervasive attack-defend and withdrawal-demand patterns between them and the fear-shame dynamic that rules their relationship that must be changed, a fact lost in the fog of dueling diagnoses.
While self-help authors who encourage readers to diagnose their partners are out to sell books, I suspect that many therapists who diagnose without examination are trying to counter the propensity of some clients to blame themselves for the poor behavior of their partners. “It’s not your fault!” is the mantra of the poorly trained. But there’s little doubt that the utter powerlessness engendered by blaming will keep their hapless clients in treatment for quite some time, to the financial benefit of the therapist.
Personality disorders are extremely complex diagnoses that only a professional should make after thorough examination and testing of the subject, supplemented by interviews with the partner. The diagnosis cannot be made by a self-help book or advocate or by a therapist going exclusively on the highly subjective descriptions of a hurt, distressed, or resentful partner.
Almost always, the motivation to diagnose loved ones is not to sympathize but to blame from a position of moral superiority—a powerful driving force in the age of entitlement. If you have an urge to diagnose your partner, you cannot at the same time have genuine self-compassion, which would give you insight into your pain, along with motivation to heal and improve; blame neither heals nor improves. Neither can you experience the genuine compassion for your partner that would give you insight into his/her deeper experience, which is necessary to evaluate the possibility that he/she can heal and improve sufficiently to repair your interactive pattern. The urge to diagnose makes you see yourself and other people too superficially. It makes it impossible to see others apart from how you feel about them, which you are likely to find on some Internet checklist of personality disorders.
You do not need to diagnose your partner to evaluate your relationship. All you need to determine whether it is viable and repairable is to feel as much compassion from your partner as you give and to know, through binocular vision—the ability to see two perspectives at the same time—that your partner sees your perspectives as equal to his/her own.