Why do some women act arrogant?

I know this girl that everyone considers really beautiful. She's pretty arrogant and stuck up though, even though she doesn't have a boyfriend. She seems to have a coolness and sense of superiority when she talks to people who are not her friends (so basically strangers). I'm just wondering what the psychological explanation for this is? I hope that doesn't sound like stupid question. Is it because she is insecure?