Any good answer for this should be personal. I assume that you should already have some experience in the pharmacy if you got an interview. What about working in the pharmacy made you want to pursue it?
Agree with the above poster. No one else can tell you why you want to become a pharmacist. It has to be personal. Think about what pharmacists do and somehow draw a pathway all the back to your core values.
Probably the best answer (subjective to me of course) would be if I heard someone tell me about their real-life experience of pharmacy and what they both liked and disliked about it and how they are interested in the problems with pharmacy. I'd love to hear thoughts about how these issues can be addressed... I don't necessarily care if you don't know the answer but that you have put some thought into these things. For one it shows a problem-solving mentality. For another, it shows you're not naive and you recognize that there are tons of issues with the profession.