Megalomania is sometimes associated with bipolar disorder

Megalomania is  sometimes associated with bipolar disorder - a depressive illness that is characterized by mood swings from extreme lows to extreme highs. During the latter cycle, people often suffer delusions of grandeur and feelings of infinite capability. They talk about unrealistic plans and goals as if these plans and goals are within their grasp.

Megalomania is a psychopathological disorder characterized by delusional fantasies of power, relevance, or omnipotence. "Megalomania is characterized by an inflated sense of self-esteem and overestimation by persons of their powers and beliefs.

A quotation by Bertrand Russell gives his interpretation of megalomania: "The megalomaniac differs from the narcissist by the fact that he wishes to be powerful rather than charming, and seeks to be feared rather than loved. To this type belong many lunatics and most of the great men of history."

Edmund Bergler, one of Freud's early followers, considered that '. . .  the child lives in a sort of megalomania for a long period; he knows only one yardstick, and that is his own over-inflated ego ....Megalomania, it must be understood, is normal in the very young child'. 

 'in addition to its pathological forms, megalomania is a mental behaviour that can be used by any individual as a way of coping with distress linked to frustration, abandonment, loss, or disappearance of the object' in everyday life. In this sense, we may see 'megalomania as an extreme form of manic defense...against the anxiety resulting from separation from the object'. [Choodle-Umma: and I can well believe this happens when a child feels rejected by its mother! What future problems are built  when a father alienates his children from their mother, especially when that father's had a bad relationship with their own mother.]

Unfortunately, 'a person with megalomania may not be interested in self-reflection or personal change', so the talking cures may be less effective than medication.

And, of course, since the delusional meglomaniac does not believe or see there is anything wrong with their behaviour, attitude or words they will never change and will continue to arrogantly order others to 'do as they are told!' and to throw their weight about.