There is no 'right' answer to such a question. Like many interview questions, it is asked so that the interviewer can establish whether or not you have thought carefully about the course of action on which you are embarking. If you haven't, it will show - and your chances of passing the interview will be correspondingly reduced.
So - if you want to know why you want to be a pilot, go and sit quietly, think about it, and write down your answers. Leave the answers for a day or two, go back to them and read them to yourself. Refine them if necessary, and then file them somewhere. You can use this process to remind yourself why you're doing this in the dark days when debts and PFOs seem to be never-ending!
Scroggs