7 years later...
The majority of stalkers have severe personality disorders. The most common one among stalkers is narcissistic personality disorder, which gives stalkers an exaggerated feeling of self-worth and an obsessive desire for other people to admire and revere them.
Extreme dependence, in which a person needs constant help, attention, and acceptance from others, and borderline personality disorder, in which a person has unpredictable emotions and a high sensitivity to rejection and abandonment, are two more personality disorders that experts often see in stalkers.
Pick one.