There is a vast difference between someone expressing emotions and the symptoms of Borderline Personality Disorder (BPD). Of course, we all experience a range of emotions. Borderline Personality Disorder is a clinical diagnosis made when someone has a profound disturbance in their personality and behavior. The official diagnostic criteria are as follows:
A pervasive pattern of instability of interpersonal relationships, self-image, and affects, and marked impulsivity beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following:
(1) frantic efforts to avoid real or imagined abandonment.
(2) a pattern of unstable and intense interpersonal relationships characterized by alternating between extremes of idealization and devaluation
(3) identity disturbance: markedly and persistently unstable self-image or sense of self
(4) impulsivity in at least two areas that are potentially self-damaging (e.g., spending, sex, substance abuse, reckless driving, binge eating).
(5) recurrent suicidal behavior, gestures, or threats, or self-mutilating behavior
(6) affective instability due to a marked reactivity of mood (e.g., intense episodic dysphoria, irritability, or anxiety usually lasting a few hours and only rarely more than a few days)
(7) chronic feelings of emptiness
(8) inappropriate, intense anger or difficulty controlling anger (e.g., frequent displays of temper, constant anger, recurrent physical fights)
(9) transient, stress-related paranoid ideation or severe dissociative symptoms
An individual with borderline personality disorder essentially has no sense of self. Their identity is largely derived from the relationships in which they're involved. Therefore,when they perceive any threat to a relationship, they can quickly decompensate, becoming verbally abusive, suicidal, self-injurious, and even homicidal. They often experience mild psychotic symptoms as well, hence the name borderline personality disorder. The term was derived many years ago when clinicians tried to classify people who were on the borderline of psychosis and neurosis.
The fact is all human beings have some traces of the personality disorders. Most of us have a little sociopathy, borderline, and narcissism built into us. In fact, we need these characteristics to survive. For instance, without any sociopathy, it would be very difficult to walk down a major city street and not become despondent at the sight of homeless people. Our innate traces of sociopathy put a barrier between us and them. Personality disorders are profound disturbances in one's ability to function, work, and relate to other people.