Why do you want to leave your current job? The canned answer that you may get when you ask why someone wants to leave his current job may sound something like this: "I've gone as far as I can go in my current job. I've learned everything there is to know about the position, and unless someone leaves, I will never get promoted."
You need to counter with a qualifying question: "What makes you believe that this job with my organization will be any different than your current job?" Listen very carefully to his answer. It better have substance. If he tells you it's because you have a dynamic organization and he thinks you are great, watch out. But, if he identifies specific aspects of the job that would be challenging, then it may be worth your while to go on to the next qualifying question.
Excerpted from Attracting & Rewarding Outstanding Employees