Failure is hard to live with sometimes. Peter is failing in this verse, right at the time when the chips are down. Jesus predicted that he would do so. What's interesting about that part of the story being found in the book of Mark is that many scholars believe Peter may have actually sat with Mark to create this Gospel. If that is true, then Peter is choosing to tell this part of his own story because it matters. Failure matters, not because it's something to get past quickly, but because failure has its own set of lessons.