You bake a chocolate cake. When you go to cut it half of it is gone. The chocolate-faced thief denies the theft. Why would you buy the lie?

Why would you NOT believe the evidence/proof/your own eyes and instead believe the insulting lie of the thief? Is it a mental dysfunction that causes that kind of truth avoidance? Is a wish that someone is telling the truth so strong that the mind can be clouded and fooled by words that are untrue? Anyone have a clue?