Sometimes people start off on the wrong foot with this question by saying negative things about their current or past job.  You need to avoid doing this.  What you want to do is explain all of the positive reasons why you are interested in a new position, specifically the position you are interviewing for.  Tell the interviewer what you like about the opportunity and how you will be able to positively contribute to the organization.  List the different things you will gain from this opportunity.  After your answer, the interviewer should be convinced that the new position better fits your strengths and goals.