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Dennis is extremely narcissistic, selfish, histrionic and vain perpetuated by his Ivy League education at the University of Pennsylvania. His sense of self-worth is entirely dependent on what others think of his appearance and he often suggests the best way to sleep with a woman is to make her think she may be raped, or by using his personal system, the D.E.N.N.I.S. system.
Dennis was diagnosed with borderline personality disorder by a psychiatrist and was given medication for it. He often struggles to have empathy for others. While it is ambiguous whether or not he has psychological disorders (narcissistic personality disorder and antisocial personality disorder have both been suggested), he has only been diagnosed as borderline by a professional.