The important thing to remember when responding to this question is to stay positive. Don't ever say anything negative about any of your previous employers. Instead, list one or two reasons related to your desire to learn and grow in your career. Perhaps you're looking for opportunities to contribute more than you have in the past, or you're looking for a greater challenge. Regardless of the actual reason, the person interviewing you will note that you elected to speak respectfully and professionally about past employers, which speaks volumes about your character and integrity as an employee.