This is one of the biggest cues I look for when I'm talking to a girl. There's nothing more discouraging than a chick that isn't flustered by a confident guy with good mojo. There's always an exception, but the general rule of thumb is that if she's too comfortable around you, it's usually a bad sign and it means she isn't interested. Even if she's flirting.
I think it's something that a lot of guys miss and a big reason why they end up misinterpreting signals.
I think it's something that a lot of guys miss and a big reason why they end up misinterpreting signals.