Why is it that . . .

conservatives love the story of the liberal who changed sides?

I seem to regularly come across stories in which there is a person whose criticism of Democrats is bolstered by the fact that he was once a Democrat?

Or the the-author-started-out-to-tear-down-George-Bush-but
-realized-Bush-has-been-right-all-along book.

Is this tendency as widespread on the left? It could be, but I don't seem to see it as much.