Personality disorders are dysfunctions of our whole identity, tears in the fabric of who we are. They are all-pervasive because our personality is ubiquitous and permeates each and every one of our mental cells. I just published the first article in this topic titled "What is Personality?". Read it to understand the subtle differences between "personality", "character", and "temperament".
In the background lurks the question: what constitutes normal behavior? Who is normal?
There is the statistical response: the average and the common are normal. But it is unsatisfactory and incomplete. Conforming to social edicts and mores does not guarantee normalcy. Think about anomic societies and periods of history such as Hitler's Germany or Stalin's Russia. Model citizens in these hellish environments were the criminal and the sadist.
Rather than look to the outside for a clear definition, many mental health professionals ask: is the patient functioning and happy (ego-syntonic)? If he or she is both then all is well and normal. Abnormal traits, behaviors, and personalities are, therefore defined as those traits, behaviors, and personalities that are dysfunctional and cause subjective distress.
But, of course, this falls flat on its face at the slightest scrutiny. Many evidently mentally ill people are rather happy and reasonably functional.
Some scholars reject the concept of "normalcy" altogether. The anti-psychiatry movement object to the medicalization and pathologization of whole swathes of human conduct. Others prefer to study the disorders themselves rather to "go metaphysical" by trying to distinguish them from an imaginary and ideal state of being "mentally healthy".
I subscribe to the later approach. I much prefer to delve into the phenomenology of mental health disorders: their traits, characteristics, and impact on others.