Because political parties view the world and each other in very different ways. The most eloquent explanation of this phenomena comes from Thomas Sowell in his book, A Conflict of Visions . In his work, Sowell explains how each political party regards the other as mistaken in its view of the world and describes the reasoning each party gives to explain why the other is wrong. By understanding the vision each party holds, we can begin to break down the wall that divides our nation and with a little luck begin to fix our schools. The unconstrained vision of the left holds that man can master social complexities by applying logic and morality to the common good.
Liberals see vast differences in the intellectual and moral potential of people.
Because of this, the left see surrogate decision making, economic planning and judicial activism as an essential part of centralized control and power used to advance society. As a result of this doctrine, the unconstrained believe that the presence of highly educated and intelligent people on the right that are diametrically opposed to policies aimed at the common good is either an intellectual puzzle or a moral outrage. Their response is to criticize or attack those on the right, inferring that they are opposed to the common good or are too ignorant to recognize it. This vision produces words and policies that are mean spirited and divisive. The left justify their egocentric actions because of the belief that they are right on any and all issues.