In my opinion there isn't a wrong way to answer this question unless you got dumped(too needy) or cheated on. If you are becoming a stong man and leading your relationshops, this should be an easy answer.
It could be anything from we grew apart, she let herself go, she wasn't into growing and just sat at home watching Jersey Shore all day, she didn't like giving bj's. An honest, congruent answer is usually the best in my opinion, unless you were a super needy chode.
If I get this question in field I always use it as an opportunity to screen and make the girl qualify herself. I say something like: "Well, I maintained my ambitions and continued towards my life goals and she was content staying mediocre, in a dead end job. I made a decision it would be best for my life to move on and continue towards what I want. But about you, what are you doing to free yourself from falling into a perpetual state of boredom?"
I have also said,"Lack of connection, I want a girl that's more than an amazing body and pretty face, someone that can hold a good conversation, has goals and ambitions, and willing to pursue those. What are your dreams?"