Cognitive dissonance explains why we justify bad decisions

Leon Festinger's 1957 theory of cognitive dissonance holds that people experience mental discomfort when they hold two contradictory beliefs — or when their actions conflict with their values. To relieve the tension, we don't usually change our behaviour; instead we change our beliefs to match what we've already done. Smokers convince themselves the health risks are overblown; people who paid a lot for a concert enjoy it more than those who got free tickets, simply to justify the expense.

Source: Festinger, A Theory of Cognitive Dissonance (1957)