Candidly share why you would like to leave your current job while focusing on how you will better yourself with this transition. For example, you might state, "I look forward to learning additional skills that I am not able to learn in my current position." Or, "I am excited to have a new team of co-workers to collaborate with, bounce new ideas off of, and create a great environment with. It is time for a new challenge!" Focusing on yourself will help the interviewer see that you are excited about being exceptional at your job while being respectful of your current organization.