"If you're very happy with your current job, why do you want to leave?"
If you portray that you are very happy with your current job, then they might ask you this question. I was in a very similar situation and I will include my answer as the example. You can use other reasons, but make sure it doesn't contradict you being happy.
Short Answers
"I like the work I'm doing and I like everyone on my team, but I have a strong desire to learn other aspects I can't learn in my current position."
"I have been in my current position for four years and I want to gain experiences in other areas. I love my job, but I want to grow more as a professional."
"I love to learn new things and after several years in my current position, I realized I was doing the same thing over and over again most of the time. So the only reason I want to leave my current job is to broaden my knowledge by gaining other experiences."
Long Answer
"I have been with my previous group for more than three years. I was doing the same thing on a regular basis. Although we received different projects from time to time, it was very similar to my previous work. So I had a desire to gain new experiences and learn from different products. I believe having a variety of experience is good for my career so I made the decision to leave my current job."
This is simple and direct. It answers the question without including unnecessary excuses. It also shows my desire to grow, learn more, and improve myself. This is a good indication of a good employee so using an answer like mine is a safe answer.