I've never really understood why people can't be honest with an interviewer about why they wish to become a Doctor. You can help people through more or less anything, such as teaching, cleaning roads or painting buildings - it doesn't matter because at the end of the day you're still helping people with whatever you do. You really need to expand on it. You must remember, you're most probably going to be in an interview with a mix of people, from consultants, students and then a random person such as vicar who will be judging you from all different perspectives. An answer of 'because I want to help people' or 'I've always wanted to do this from when I was born' makes no sense at all, it's not real unless you expand on it - when I was young I wanted to be an ice-cream man for goodness sake and then in the next five minutes I wanted to be a fireman because I thought they were cool... The fact is, they could easily ask you 'why didn't you want to become this or that then?'. Similarly, saying that you want to help patients and make them better, then why don't you become a Nurse? Answers really need to be honest, obviously not as honest as 'chicks, money and power' as such, but subtle and something that comes from YOU, not a textbook.
You may have had an experience of someone dying at a young age, so has most of the world and it doesn't mean that you SHOULD be a Doctor unless you can actually relate it to why you want to do the profession - 'my grandmother was ill from X and I felt just useless, this is why I wanted to become a Doctor' makes no sense, surely as I've said before, why don't you become a Nurse? You could make them feel more comfortable and help them. Wouldn't everyone feel useless, even the Doctors if they couldn't do something to help? - We must realise that being a Doctor doesn't mean that you have a cure for everything, we all have limitations and just because something happened when you were younger doesn't mean that you were destined to become a Doctor, but has influenced your decision.
My point is, an interviewer is someone that has lived through that moment of being interviewed and is expecting that cliche answer of helping people - everyone wants to help people! You need to be honest about it, expand and relate it to something so that it actually means something. That's just my opinion though, coming from someone who does want to study medicine, of course I'm probably going to get asked 'what do you know about this', but I can tell you coming from someone who reads about this quite a bit that most people want to become a doctor for the wrong reasons most of the time and need to find their true reason - not a textbook response.