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 shall think you a simpleton, for there is but one man in the world I love, and he is an angel. I should never be happy without him, so think it no harm to be off. You need not send them word at Longbourn of my going, if you do not like it, for it will make the surprise the greater, when I write to them and sign my name ‘Lydia Wickham.’ What a good joke it will be! I can hardly write for laughing. Pray make my excuses to Pratt for not keeping my engagement, and dancing with him to-night. Tell him I hope he will excuse me when he knows all; and tell him I will dance with him at the next ball we meet, with great pleasure. I shall send for my clothes when I get to Longbourn; but I wish you would tell Sally to mend a great slit in my worked muslin gown before they are packed up. Good-bye. Give my love to Colonel Forster. I hope you will drink to our good journey.

Your affectionate friend, LYDIA BENNET.

The great slit in the muslin gown is believed by some Austen scholars to be a hint about Lydia’s behavior. I don’t think she had actually slept with Wickham before they left, but it is clear that she wanted to, very badly, and it’s clear that she had allowed him certain liberties. You’ll notice that she thinks they are going to Gretna Green, in Scotland, where English couples canonically went if it would not be lawful for them to marry in England.

Gretna’s famous “runaway marriages” began in 1753 when Lord Hardwicke’s Marriage Actwas passed in England; it stated that if both parties to a marriage were not at least 21 years old, then parents had to consent to the marriage. The Act did not apply in Scotland, where it was possible for boys to marry at 14 and girls at 12 years old with or without parental consent (see Marriage in Scotland). Many elopers fled England, and the first Scottish village they encountered was Gretna Green.

So her intentions, however silly, were honorable according to the mores of her time. She thought she was getting married. They were starting from Brighton, a seacoast town in the southeastern part of England, so London was certainly on their way. Once he had her in the carriage it was an easy enough thing to suggest a stop in London. As a diversion-seeking country girl who walked to the small market town of Meryton as often as possible from her rural home, undoubtedly she would have loved the idea of a few days in London, and she was naive enough to believe that it did not matter whether they were already married.

Austen tells us, through Elizabeth’s observation and surmise, what happened.

Wickham’s affection for Lydia was just what Elizabeth had expected to find it; not equal to Lydia’s for him. She had scarcely needed her present observation to be satisfied, from the reason of things, that their elopement had been brought on by the strength of her love, rather than by his; and she would have wondered why, without violently caring for her, he chose to elope with her at all, had she not felt certain that his flight was rendered necessary by distress of circumstances; and if that were the case, he was not the young man to resist an opportunity of having a companion.

We know that Wickham felt he had to desert his militia unit because of debts — not only gambling debts to his fellow officers, but debts to the local tradesmen. We also know he was spending a lot of time with Lydia, fooling around, and we know that his friend Denny had a fair idea what was going on, so Lydia found out either from Wickham himself or from Denny that Wickham was planning to go away, and did not want to be parted from him.

We also know that Wickham never intended to marry her. He needs to marry money. But he is reckless and unscrupulous enough to welcome her along as a piece of ass during his flight from Brighton, because he just doesn’t care. Her passionate infatuation accomplishes the rest. Wickham would have left her behind in London as soon as he decided what his next step was. If not for Darcy’s intervention Lydia would truly have been lost.

Based on an answer I originally wrote on Quora.