Well I think the above are good replies, but wanted to give a word of caution. Sometimes the answers that crop up the most do so because they are the best ones. It might be cheesey to say something like you've dreamt of becoming a pilot since you were a kid, but it's certainly quite a safe answer. If you try to be too different then you might stand out - but maybe for the wrong reasons. So while it's probably a good idea to avoid the typical cliche' where possible, I wouldn't necessarily recommend trying to be entirely radical. A good philosophy is to be honest. If you are facinated by the Physics that underpin flying then say that. But whatever you do your answer should convey your passion for flying. That's often more about how you say something than merely what you say.
Hope it helps.