Why are some girls so sensitive?

I connected with a girl on a language learning app a few months ago. We used to talk casually, and everything seemed fine — we were just friends, nothing more. Today, while we were both online, a random guy joined the conversation. She started asking him personal questions that he clearly wasn’t comfortable answering. I stepped in and said that he didn’t want to answer, just to respect his boundaries.

Later, she messaged me saying that our "vibes didn’t match" and that she didn’t want to continue talking. It was a bit sudden, but I wasn’t expecting anything serious from the friendship anyway. She already has a lot of male friends, including people I’m also friends with, so it didn’t seem like there was anything exclusive or deeper going on.