Some guys just don't know how to flirt with women. What's even worse, they don't even recognize when a girl is flirting with them. They are basically oblivious in their interactions with the opposite sex.
Now, I have to admit that flirting isn't something that comes naturally for most of us. In fact, we really suck at it. But just like most things, we can actually do something about it.
The one thing women are really good at is reading your intentions. No, they aren't mind readers! They are very good at reading your body language though. That's also why you need to know certain things about your own.
If you have no idea about what your posture is telling to others, you will also have no idea what kind of stuff you might "reveal" to her.