When there are only three answers to any interview question, this one focuses on your "motivation to do the job." Making a decision to leave your current job, especially in this economy, is difficult -- and juicy information for a person doing an interview. How you answer provides insight to the hiring manager for your willingness to do the work.
I had an inside shot at a job at a Fortune 100 company and was interested in doing the work. When asked why I wanted to leave my job, my answer was that my manager was a thousand miles away and I needed permission to talk with his manager who had his office down the hall from mine. Note: not permission to talk about policy or business direction, but permission to simply talk with him. Like in the hallway on the way to the break room or something.
So I said that I thought it was overly restrictive to get permission to speak to someone with an office less than a hundred feet from mine and, with that management style, it wasn't compatible with the way I work.
My insider later told me that the answer was totally negative on my manager and since I had a bad attitude, I shouldn't get the job. While I was stunned at that response and realized my truthful mistake, I'm now happy I wasn't going to work for a manager who agreed that a permission slip to talk to someone was necessary to cover their insecure feelings about their competence.
But if you need a job, saying anything remotely negative about your manager won't help your cause.
Since this question is about your motivation for doing the work, you need to answer based on what you like about your work and how your previous position doesn't now meet your need to improve your job skills.
One approach to this is describe how your last couple of jobs have improved specific job skills -- and then how the job you are interviewing for will build on these job skills. You have to take what you have and show motivation for the prospective job at hand.
A different, but equally effective, approach is to show what you have learned from the previous job but can't learn more from it because... (insert reason here). My previous company is too small to get the specialization I need. My company doesn't have the department I would need next to improve my skills. My company decided not to expand in my job skill area so I need to move to continue building my skills.
This means that you showcase your current skills, talk about the reason you can't meaningfully improve them at your current job, and how the job you are interviewing for will support increasing your skill.
Show your motivation, get a shot at the job.