When does a girl get labeled as 'unattainable'?

What kind of behavior etc. Does girl have to do? Because somehow I get labeled as the 'unattainable' girl on campus. I discovered this by finding a chat between a lot of guys on the Internet and actually I was kinda shocked because I'm the 'shy type' of girl and try to keep myself on the background... Although this doesn't mean I dress like a wallflower I'm always dressed nice and keep myself in shape. I'm actually looking for a long term relationship so how can I get from the 'unattainable' girl to a girl guys aren't afraid of to ask out on a date?
When does a girl get labeled as 'unattainable'?
Post Opinion