This is one of those questions that, as broad as it is, you should have a well-prepared and specific answer for. You want to demonstrate to your prospective employer that you are focused, know what you want, and have a genuine passion for your chosen industry.
The most important thing is to show the interviewer that you have specific reasons for choosing that particular career path, as that implies a dedication and enthusiasm that will motivate you and get you far. You could point to aspects of your personality, interests or significant experiences from your personal or professional past that have helped to steer you in your present direction. Citing reasons of money, status or that your parents made you do it won't do much to advance your cause – demonstrating genuine passion will.
Show the interviewer that you and the position you are applying for are a perfect fit. If you are applying for a job in public relations you could say, ‘I’ve always been a strong communicator and people person. I also like a fast-paced, high-energy environment so a career in PR is just a natural path for me.’ Alternatively, if you are applying for a design job you could mention your excellent eye for detail and aesthetics, and your life-long fascination with all aspects of design. Perhaps you have been sketching and working on personal creations from a young age, and can name certain designers or mentors who have inspired and influenced you.
With such an open-ended question, be specific but refrain from rambling on too much. Don’t give long and boring answers – be direct and to-the-point. Pick one main reason why you chose the career path and elaborate with a few specific points (keeping in mind the job requirements). Show the interviewer that your skills and personality are exactly what their company needs.