Influence: The Psychology of Persuasion
It is, quite simply, our nearly obsessive desire to be (and to appear) consistent with what we have already done. Once we have made a choice or taken a stand, we will encounter personal and interpersonal pressures to behave consistently with that commitment. Those pressures will cause us to respond in ways that justify our earlier decision. We all fool ourselves from time to time in order to keep our thoughts and beliefs consistent with what we have already done or decided.