system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] What did the Joker say to his psichiatrist that made her become Harley Quinn?
gpt
He manipulated her into thinking he was her friend, then into thinking he genuinely loved her. Harleen never really had friends and never formed any strong attachments to anyone, so when someone seemed to be showing her genuine affection, she fell head over heels. It didn't hurt that she felt that if she successfully treated the biggest monster in Gotham history, she'd be famous, so she threw herself completely into his world to learn his "secrets". 

In some universes, when she talks to the Joker for the first time she misinterprets what he's talking about entirely. He talks about meeting his equal, his better half, someone he truly understands and who understand him. She assumes, since she's the only other person in the room with him, that he's talking about her, especially since he speaks so warmly and with so much conviction. He's of course talking about Batman, but he never explicitly says so. However, he very quickly realizes she made that mistake and continues to intentionally mislead her so she eventually does whatever he says so he can escape. And by then she's at the point of never leaving his side.