Why is my best friend so flakey and insecure?

One of my best friends is really insecure she would criticize her own height, even though she's a perfect height for a girl and there's this guy she seems to really like but once he asks her out she just flakes out on him and I feel really bad for the dude. She also left me for a while (whilst still talking) for another group of friends and left me by myself for a while. I think it had something to do with the guy she likes to be honest as she has started to ignore him and act pretty cold to him. I've asked her whether if he was still bothering her but she didn't want to talk about it. It's kinda crazy because I know she likes him a lot but instead of literally saying yes to him she'd keep checking him out and starring at him awkwardly and then looking away. In my opinion her behaviour is self-destructive and immature. Why is she so flakey, immature and insecure? How do I help her?
Why is my best friend so flakey and insecure?
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