Why does Elizabeth blame herself for Lydia's indiscreet behavior and loss of reputation?

Elizabeth has no doubts that Wickham does not plan to marry Lydia. She knows that Lydia would not have gone off with him if she were not under the pretense that they were going to be married, but Elizabeth also realizes that Lydia is easy prey for Wickham's deceptions. Elizabeth feels responsible for introducing her to Wickham in the first place.