How about..."Because I bothered to show up for this interview?"

Kidding.

Think about your personality in relation to the public and customer service and anything you've seen and liked when you've been in the place. Do you like the challenge of making a customer happy? Does it make you feel good to be part of a team? Avoid stuff like being a great leader and someday owning your own franchise...The interviewer wants to know how your particular talents and personality can help him now. Really, all he's looking for is to avoid potential problems. Keep it simple. This isn't really a hard sell.

Also ask yourself (not the interviewer ) why you would want this place to be your employer? Perspective. And don't go in feeling like this is the only offer you'll ever get. Good luck.