There are several things the interviewer wants to learn from this question. The interviewer knows you are looking for another job because you are not happy with where you are at. Knowing this, they want to find out how you react to situations where you are not happy. Most people who do not prepare for this question will have a generic answer that doesn't stand out. You can use this opportunity to give a solid answer.
Despite having some negative feelings about work, this answer shows that he or she will continue to do hard work regardless of being unhappy. You shouldn't say you are unhappy, but looking for another job is one indication of not being happy.
Learn English speaking much FASTER by downloading the TalkEnglish Offline Package and submerge yourself with over 8,000 audio files and over 800 pages of lessons! Then you can study without Internet connection and also listen to audio files using your MP3 player at any time. Go to the English Download page and download today!