In Pride and Prejudice, why did Wickham run away with Lydia?

Because she was willing to go.

Wickham’s actions were careless and contemptible, and made even worse by his lack of authentic affection for Lydia. But I believe that he did not have to do much persuading.

Lydia is a spoiled rotten youngest child, heedless and shallow, obsessed with beating her older sisters to the altar. She is also dangerously naive, and she lives in an era of rigid social rules, so she assumes that in eloping with him she is automatically going to get married. She’s 15, has been boy crazy as long as we’ve known her, and is in the throes of sexual obsession. She may have still been technically a virgin when Wickham bundled her into that carriage, but we have a suggestion that they have been as far as third base together, so she’s just discovered sexual arousal, and most of us still remember what that discovery was like. It’s, uh, very compelling.

Here is the text of the note Lydia leaves for her intimate friend Harriet Forster, the very young wife of the regiment’s commanding officer, Colonel Forster:

MY DEAR HARRIET,

You will laugh when you know where I am gone, and I cannot help
laughing myself at your surprise to-morrow morning, as soon as I am
missed. I am going to Gretna Green, and if you cannot guess with who,
I…