The mind of an imposter

Every time I read one of these things, I wonder at what motivates someone to make up an entire career, a whole set of credentials. Part of it is almost admiring, since I'd never have the chutzpah. But more of it is just curious. Why do it? Is it the middle-class equivalent of a fourteen year old kid stealing a minibike--unable to conceive of getting the credentials through legitimate means, they simply help themselves to the unearned when no one's looking? Or is it something more sinister, some positive enjoyment of the fraud?