If your answer is because you hate your boss, you just lost this opportunity! No employer ever likes to hear negative discussions about your current or prior work engagements. Any negative discussions will be portrayed as "red flags" to the employer which are not good for you. Depending on your circumstance there could be many answers to this question. Simply give a positive response with a logical explanation such as "I decided to relocate so that I could be closer to my family" or "I've been at my current employer for 8 years and I've reached my maximum potential, so I've decided to search for a new and challenging opportunities", etc.