The easiest way to answer this question is to respond with what initially compelled you to apply for the position in the first place.
Did you like the job description itself and then, after reading more about the company, were attracted to their mission statement?
Or have you spoken to current employees and noticed everyone seems genuinely happy working there?
Whatever drew you to the position is what you should answer with here.
But you should also be prepared to take it a step further.