Interesting question. I have to say it's hard for me to explain *why* hair, I can however explain *what* makes long hair is so beautiful
Beautiful, long, shiny, healthy hair completes a woman. The way it flows over her shoulders and accentuates her face is simply amazing. To me healthy, well taken care of hair also reflects how she takes care of herself in general. A woman with beautiful hair shows that she's proud of herself as a person and comfortable with her body and appearance. It takes time and effort to have good looking hair, so when she puts in this time and energy it's an indication of her being someone who takes proper care of herself.
The way a woman handles her hair, the way she flips it or tucks it behind her ear can be really arousing. To me it's very very sexy when a woman casually flips her hair while talking to me.