People who live or interact with each other tend to have similar attitudes. In the case of Bennington College students came from wealthy, conservative families. However, at this liberal college each year they became more and more liberal. We essentially refuse to listen to things we disagree with. This supports Cognitive Dissonance.
People also think that they can change their attitudes to justify past actions.
There is also an insufficient justification effect - defined above. They must also perceive their action as stemming from their own free choice. The justification action is strongest when the action is felt to be justified. Skinner said that attitudes are verbal habits. They play a role in what we say, not do.