Why did you leave your last job so abruptly? Or why do you want to leave your current job? How do I know you won't do the same here?
For more than thirty-one years that I've been in this profession, this is one of the most difficult and treacherous questions you're going to be asked. Underlying this question is a concern on the part of the hiring authority that whatever reason you used to leave your last job or are using to leave your present one, you're going to use when you leave them. Depending on the state of the economy, a hiring authority is going to be more sensitive to the answer you give to this question. When the employment economy is robust, hiring authorities are more concerned about what you can do for them immediately than they are concerned about why you left your last position or why you want to leave your present one. This was true in the early 1980s and the late 1990s. When the employment economy is more difficult and there are many candidates to choose from, most hiring authorities are more concerned about the reasons you might have left your last employer or why you're looking to leave your present one. No matter what the economy, all hiring authorities are very concerned about the answer to this question. But, the answer is more important in a difficult economy than in an expanding one.
The biggest mistake candidates make when it comes to this question is that they, first, don't mention how much they appreciate the job they had or presently have and then, second, talk about selfish, or self-centered, "me" reasons for either leaving or changing jobs. Anything related to what you wanted when you left or want to leave will not fly. Answers like, "Well, I wanted more money (or a bigger company or more prestige)," anything related to your needs, before the needs of the company you were with, will destroy you. Remember, whatever you say, the hiring authority you are talking to will take it personally and envision a time when you will say the same thing about him and his company.
So, you have to find a relatively "antiseptic" answer to this question that neutralizes any kind of negativity. Something along the lines of, "You know, I really loved that job for_______ and really appreciated all of the people who I was working with. Unfortunately, the company had to downsize because of the economy and since I was one of the last hired, I was one of the first to go." Or, "I really love my job and I love the people with whom I work. Unfortunately, I have reached a point with my position that the opportunity for me to grow both personally and professionally just isn't there. They are good people and have been good to me. I will certainly miss them, and I'm sure that they will miss me, but I am capable of greater responsibility and authority and the odds of me getting that where I am just aren't very great." You absolutely have to spin the answer to this question into a positive.
Never criticize, denigrate, badmouth, or in any way speak poorly of your present or past employer. If you can't say something good, at least make it neutral. If you try to make your present or last employer out to be a "bad guy," no matter how justified you are, a hiring or interviewing authority will never, ever buy into your excuse or reason.
The answer to this question kills more opportunities for candidates than probably any question that can be asked. You need to really think about whatever answers you are going to come up with and put yourself in the shoes of the interviewing or hiring authority and ask yourself, "If I didn't know me as I do and I was being compared to a number of other equally qualified candidates, how does the answer to this question make me look?" If the answer to this question makes you look like a dedicated, committed, reasonably well-performing employee, who for very good business reasons is looking for a job, you'll be fine. If your answer does anything less than that, you're dead in the water.