What causes someone to be overly obsessed over someone they literally just met?

There was a guy working on our cameras at Dollar Tree where I work. Well, my friend, She thought he was so cute, and she kept making loud remarks about how cute she thought he was. She was acting extremely twitterpated, even saying stuff like she thinks he would look good in bed. And she said he can do whatever he wants to her.

I was embarrassed because I don't think it's appropriate to act like that in front of a guy. She was too shy to give him her number, so I ended up giving it to him for her. The only reason I gave it to him, was because I hoped it would help her calm down. (She is in an awkward situation with her boyfriend, they keep breaking up and getting back together, but she literally had sex with him last week.)

He messaged her, and ended up telling her that he had a girlfriend. But that he thought she was gorgeous. Which I think is kind of weird that he told her that if he has a girlfriend, but anyway. She told him that she thought he was so so handsome, after she knew he had a girlfriend. Well, then I was talking to her two days later after this happened, and she told me that she is so heartbroken that he has a girlfriend. Which is confusing to me because she only saw him for like 5 hours. So I don't understand how she's heartbroken over a boy she only saw for 5 hours and never met or even talked to in person. Then I saw she posted on her tik tok and said "the heartbreak of liking a guy with a girlfriend".

Why would someone be so obsessed with someone like that?

What causes someone to be overly obsessed over someone they literally just met?
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