A good answer would be the real reason for that individual, an interesting answer would be one I have not heard before or expected from the candidate - however this may not always be the same as the good answer.
Neill is right, (ouch that hurts!), but some example of good answers might be that "I am curious, detail oriented, and have a development background, but would rather enjoy the adventure of analyzing software and digging for answers." Or maybe "I have always been good at taking things apart and think I would enjoy software testing, it appears to offer a real challenge."
An interesting answer might be "I personally feel that developers have pompous attitudes and are egotistic, and I would really enjoy bringing them down a notch or two by finding bugs in their code!" But I wouldn't advise using the last one. The interviewer might be an ex-coder!
Agree with Neill. When I started, my answer was "I don't want to be a tester, I see it as a step in developer career". I'm in testing for almost 10 year since then and I love it for reasons that are hard to explain in a single sentence (more over I know the reason only now, as many years have past).