Personal gullibility and vulnerability are frightening. We can't believe we are capable of being lulled into believing something that isn't true or that we trusted an untrustworthy person. We don't want to have to admit that we can be manipulated. It takes brutal honesty and courage to stare at our own dark side, let alone to confess it. The truth that frees also hurts.
Denial looks like less work.
We like the person and want to believe what is being said. We want to go back to our job, our families, our lives. We are taught to trust. We teach our children that trust is a good thing. Trust is the best gift one spouse gives the other.