Time usually flies Business class I think...
My real answer would be that time doesn't fly in the sense that people describe it to in the popular saying... Only our perception of the passage of time actually changes, usually in relation to our enjoyment levels at the time.
So time appears to drag when we are doing something we don't particularly enjoy and are wishing taht it was over and done with, and it rushes us by when we are doing something we love and want to keep doing it for as long as possible.