Why did Peter deny Jesus three times before the crucifixion?

Les explains that leading up to the crucifixion, when Jesus was arrested and a young maid thought she recognized him, saying 'You're one of them,' Peter denied it three times and swore. Les says the reason was that he was afraid of the powers that were. But Les always makes the point that after Peter witnessed the resurrection and its power, he became a different person—no longer afraid of anybody. He stood up to the Jewish religious leaders and the Romans, and no doubt took a martyr's death because of it. Yet later, as an ordinary practicing Jew in a moment of weakness among Gentile believers, he blew his testimony again, and Paul had to call him on it. Other Jews dissembled or withdrew likewise with him, even Barnabas was carried away with their dissimulation or hypocrisy, so they couldn't eat with these Gentiles.