Why are you leaving your current job?
Like many interview questions, there are lots of potential subtexts that might start running through your head when you hear this one come out of an interviewer's mouth. Are they trying to get me to dish the dirt on my current employer? Have they heard something? Can I tell them that I'm bored and underpaid?
Here's the truth: none of these things is what your interviewer is looking for. What they are looking for is evidence that you've thought this through. Yes: they know you want more money. Everyone wants more money, with two exceptions: people who are looking for a better work-life balance and those who will take any port in a storm. It's the latter person that employers are trying to screen out—not because they don't want someone who is fleeing a bad situation at work, but because they want to be sure that, if they hire that person, they're not going to turn around and quit six months later when the offer they really wanted finally materializes.
So what's that got to do with you, and how you approach your answer? All you need is a good rationale for why you want to join the new firm. Which means that the key to answering this question isn't about what's wrong at your current job: it's about what you want to do at the new one.
Consider the following two answers: 1) "I feel like I've gone as far as I can at my current job and am looking for a new challenge," and 2) "I really want to concentrate on business analytics, and I know that that's a big focus here. In my current role I have a limited amount of ability to practice analytics, and so I'm looking for an opportunity where I can continue to develop that skillset and put it to use."
See the difference? The first one explains why you want to leave your job but doesn't tell your potential employer anything about you. The second answer, though, does both: it explains your current frustration in a professional manner and also clearly communicates that you've done some research about the company, know what you can bring with you, and are excited about the possibilities offered by the role. That's exactly what an employer wants to see in a potential hire.
The key to answering this question—like most others—is to do some preparation. At a minimum, you should have enough knowledge about your target firm to be able to spell out how you'd see yourself fitting in. You should also be able to explain how the opportunities in the new role match what you're looking for, and use that as the point of contrast with your current employer. If you can manage that, you'll have taken another step towards leaving your current job behind—regardless of the reason why.