I was about half in love with her by the time we sat down. That's the thing about girls. Every time they do something pretty... you fall half in love with them, and then you never know where the hell you are.

This quote is talking about how quickly you can fall in love with someone. It is saying that every time you see a girl do something that is special or beautiful, it feels like you are falling in love with her. But then, when you stop and think about it, you don’t really know how you feel about her anymore. It is like you have no idea how strong your feelings really are.