Why do people manipulate? There are many reasons and none of them are justifiable. Here are some of the more obvious and common reasons:
* To get their own way
* To prevent you from making a free choice
* To feel in control and powerful
* To impose their reality on you
* To avoid responsibility
* To gain an unfair advantage
* To distort reality
* To preserve their distorted self-image
* To diminish you
* To discourage intimacy
* To coerce
These goals are only examples, sometimes a person may manipulate simply from habit. Habits become ingrained; manipulators reach for the familiar tool rather than exploring the situation and choosing the most effective tool. In "Habit, Chapter IV in his Principles of psychology, William James (1890-1981) made one of modern psychology’s first pronouncements on the course of adult human development.
Already of the age of twenty-five you see the professional mannerism settling down on the young commercial traveler, o the young doctor, on the young minister, on the young counselor-in-law. You see the little lines of cleavage running through the character, the tricks of thought, the prejudices, the ways of the "shop", in a word, from which the man can by and by no more escape than his coat sleeve can suddenly fall into a new set of folds...In most of us, by the age of thirty, the character has set like plaster, and will never soften again. (Pp.125-126)
Fortunately, that is not quite true, but for the person with a personality disorder or severe maladaptive personality traits (without fully meeting the criteria for a personality disorder) substantial change is unlikely.
You have power and you can use that power and healthy behaviors to handle manipulators once you learn how to recognize them and their behaviors. We do exactly that in the Manipulative Man.