After we have made a decision, we will feel dissonance regarding the possibility of it being wrong. We will often change our perceptions to reduce this dissonance and make the decision seem more attractive.
This is the basis of the foot-in-the-door technique where people who are asked to make a small commitment (such as signing a petition) will later change their views to align with the action and consequently be more amenable to a more significant request. It is also the basis of brainwashing.