It is a very special question because the interview wants to know your motivations. A sample of bad answer might be "For money" this answer could lead to misinterpretations, for example that you do not want to work in the company to contribute with your abilities, but only for the money. Remember that interviewers want to hear responses, in which the main motivation is personal growth. A sample of good answer might be:

I left my last job because there, the company does not have opportunities for personal growth.