The thing is with women they know that men are mainly concerned with looks whereas women generally tend to look for more than that.

They'll be worrying about what you think of them and if your worrying about what they think of you then it's just going to be one big awkward mess.
When you are too in your head she might be thinking 'this guy is creepy why do I attract creepy guys' or 'he's not talking that much or showing much interest am I ugly or he's not into me'.

Whereas if you are just uber cool 'I don't care what you think' kind of person it's going to put her at ease too, she'll think 'a fun awesome comfortable guy is talking to me I must have some value'.