How to Address the Question - Why did you Leave your Job?

When asked "Why you left your position?", you don't want to raise additional questions. You can say that you had accomplished everything you could with that particular employer and it was time to grow forward OR that you realized that the XYZ field was not for you so you were changing career direction. Let them know how THEIR job has what you are looking for (what was missing out of the job you left). Keeping the conversation in a positive place will emphasis your professionalism. Professionals move on and don't blame. Taking a stance where you needed to move on for growth or advancement or to truly focus on your career can be both true and positive, thus very professional.

Source - Marianne Grady (as posted on JibberJobber)