Histrionic Personality Disorder is currently defined - in the US 'Diagnostic and Statistical Manual of Mental Disorders' DSM5 - as :

A pervasive pattern of excessive emotionality and attention-seeking, beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following:

• is uncomfortable in situations in which he or she is not the center of attention
• interaction with others is often characterized by inappropriate sexually seductive or provocative behavior
• displays rapidly shifting and shallow expression of emotions
• consistently uses physical appearance to draw attention to self
• has a style of speech that is excessively impressionistic and lacking in detail
• shows self-dramatization, theatricality, and exaggerated expression of emotion
• is suggestible, i.e., easily influenced by others or circumstances
• considers relationships to be more intimate than they actually are

The cause - or causes - of HPD are unknown.

Little research has been done to find evidence of what causes histrionic personality disorder. Although direct causes are inconclusive, various theories and studies suggest multiple possible causes, of a neurochemical, genetic, psychoanalytic, or environmental nature."

Source : Wikipedia