Why do girls flirt just for fun?

A girl that I like flirts with me a lot, but it's only for fun. Every once in a while she'll go out of her way to get my attention, tease me, and even getting her friends in on the action, but never anything too serious. How should I go about showing this girl that I don't want to play games with her any more and instead that I desire a more sincere relationship with her?
Why do girls flirt just for fun?
Post Opinion