When an employer ask “why do you want to change your job?”, he is naturally curious of why did you leave your last job. The reasons tend to be more personal than not, something like “I’m left because I can’t stand my co-worker and the boss bullies me a lot”, if so, you don’t want the interviewer to know that. These kinds of answer just shows immaturity and that you might be “hard to work with”, putting your chances in a very bad spot. You might want to tone down and make it more formal. While some employers might sympathize with you, I would rather you to not take that risk. The last thing you want is the employer seeing you as the “problem” employee.
You can try this simple format for answering the question:
Start with something positive about the job you left, something like “I’ve work with wonderful people and learn a great deal”. You want to assure the employer that you are not just escaping the last job or that you are problematic.
Then follow it up with what makes you interested in your new job. “I’m interested in the opportunity that you are offering and I have always heard great things about your company. So your company is the logical next step for my career.” Focus more on how you will be the best person for this opportunity.
Lastly, a closing statement that will reflect nicely on you. Maybe flatter the company more. “ I’ve always been a huge fan of the company’s services and so I would love to contribute to its success.”
The point is, avoid speaking ill of your old job and show more that the new job is the right one for you. The interview is all about leaving a lasting impression. Keep the answer short, sweet and directed towards the future with the new company, rather than the past employment.