There are many wrong answers to this question. Some of them include saying things like, I hate my job, I hate the company, I'm not appreciated there, I hate my boss, I'm sick of working there, etc. Basically, anything negative is a bad answer. If you say something negative, they will think you will eventually get into that situation again while working for them. So I highly recommend you have a good answer that leaves a positive impression while displaying good traits.
Answers should be like this:
"I want to find a company with more opportunities. My previous company was very small and didn't have opportunities for growth. After learning about the work environment here, I felt this is exactly the type of place I want to work."
"My current company is very large and it's difficult to do different tasks. We have routine work that never changes. I want to work in an environment where I can utilize more of my skills."
"Our family moved to live near a better school district for our children. This area is great and I want to find a job closer to my new home. My previous job was too far away to commute."