Imagine you introduced yourself as John Smith. And someone overhears you and says, "No you're not."
"But here's my friend who can vouch for me."
"He only knows who you say you are."
"Here's my driver's license."
"Here's my mother who gave birth to me!"
"Babies have been switched in hospitals."
How long would you go on trying to convince this person that you are who you say you are?
Fast successions of quick denials are a surprisingly effective way of attacking a proposition. This is particularly true if the proposition is even slightly complex.