Interviewers ask this question to determine if you will be a good fit for the company culture and to ensure their organization can meet your needs. They also want to see if you are honest enough to admit if you were fired or let go for another reason. Talk to the hiring manager about what is causing you to leave your current position. Be careful this does not turn into a conversation that bashes your current employer, colleagues, patients, or managers. How you answer this will tell the interviewer a lot about your character.