Guys often misread the signals from girls who are just being friendly, just doing their job, giving good customer service, or being flirty within the safe confines of the customer/service provider relationship.
I love this scene from the movie Adaptation; it pretty much sums it up.
It's one of those awkward moments a lot of us can relate too.
https://www.youtube.com/watch?v=r0MPwCYmFQo