Why do you want to be a pilot?
"I became a pilot because I went on a trial flight and I loved it. I want to fly for your airline because x,y and z."
Where X, Y and Z relate to that specific airline and what you love about flying.
This not only shows that you love to fly, but that you've researched the company and know a bit about them, and that you believe you will fit in with their culture.
They're not asking you why you want to be a pilot, they're asking why they should hire you.