The question of motivation is one of the easier questions the interviewer asks to get a better idea of your values and personality and therefore assess whether these coincide with the company culture and nature of the position you're applying for. Be honest, but make sure that this is relevant to the job and company you're interviewing for. A safe response is to say that you're spurred on by challenges to learn and continuously improve yourself. This article on About.com has more suggestions on how you may want to phrase your answer to this question.