As in, you meet a girl, you start flirting, vice versa, and you don't know when to ask her out?

Well, what you need to do is talk to her about it. Make she would want a BF, some girls flirt around, but don't want a relationship at the time. If she wants a BF, then go ahead and try. It's better to be rejected and know you tried, then having that regret knowing you didn't do anything. Like I've said before, communication is key. Also, if she is scared of ruining the friendship, she may not be what you want. You need friendship before you can have a relationship. You gotta take risks. Just talk to her, see if she would like a relationship, you know, hint around a bit. Some girls like it, some don't. Some want you to straight out say "Wanna be my GF?" So, good luck man.

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