A Similar question was posted a few months back - 'why do you want to be a pilot'. The answers tended to be along the same lines, basically "I cannot put it down in words, I just do".
It is a nightmare question at interview but maybe honesty is the best, 'I just want to fly but cannot answer why, it is an instinctive and overwhelming feeling'.
You cannot second guess the interview panel and they have heard it all before anyway. They would also probably be hard pushed to answer the question themselves.
