Interviewers ask this question to determine whether you've put enough consideration into such a big decision and why you want to leave your current job.
Are you genuine about changing your job, as the employer doesn't want someone who is half-hearted about such a serious decision. You need to make sure you come across convincing.
When answering this question, avoid saying anything negative about your current employer's company or that you have issues and matters unresolved with your teammates or superiors. It's never a good idea to belittle current or past employers, as the interviewer might think that you are either a complainer or wonder if you are the problem.
Have a planned and genuine reply for why you desire to leave your job. Some of the reasons can be that you honestly are looking for a new challenge in your career, learning a new industry or focus area, or needing to find a new job because you're relocating.
When answering this question, be prepared, honest and positive.