One theory on how the various attachment and personalty disorders occur is that the natural process of development during infancy and toddlerhood is arrested. During infancy --> toddlerhood we begin our sense of reality with only a sense of self. Everything, including one's mother, is simply an extension of that awareness of self, but there is no comprehension that others exist independently of our comprehension and needs. During healthy development there is a gradual process where the infant begins to comprehend that their mother is a separate being with an independent set of needs. This is why very young children will cry for attention even if the mother is sick, in the shower, busy cooking food, etc. The early mind cannot comprehend reality from a second viewpoint.
Even young children in the pre-operational stage of development cannot view the world from another perspective. They have done tests placing young children around a table with various shaped objects in the center of the table. When the child is asked to draw the objects as they would appear from another child's perspective, they cannot do it. This is also seen if a young child nods 'yes' while talking to grandma on the phone. They cannot process that grandma cannot see them.
During healthy development, children can go through the process of learning empathy, which can include projecting their own feelings onto their stuffed animals and other people. Gradually, they develop theory of mind in which they can comprehend what another person is experiencing and feeling even when this is different from their own experience.
When an individual's development is arrested psychologically and emotionally at these young stages, they still grow into an adult who is capable of abstract thought, long-term planning, and complex calculations, but still incapable of affective empathy. They cannot feel from the perspective of others. Some do develop a type of theory of mind (although I suspect it is constructed differently than the healthy version) in which they can be skilled at predicting behaviors and vulnerabilities in others without having any shared, affective empathy for others.
So these people experience the world much as you or I would experience living in a holographic simulation of reality. Nothing besides the self is real. Nothing besides the self matters. Everything exists to be manipulated and used to satisfy the impulses of "mine! mine! no! no!". The toddler level of emotional processing is combined with abstract thinking of adulthood. Harm done is not experienced as regret because the effect it had on others does not matter and isn't comprehended as 'real'.
