While I agree with L2D that this is a preallo question, bite anyways. I'm not going to give you my answer because I don't think it will help you in anyway, instead I'm going to tell you whats wrong with your question and hopefully some ways that you can think about it to come up with your own answers.
Ok, so you want to help people, well, why not nursing/firefighting/PA/NP/policeman/civil rights lawyer/politician . . . all these people help people, this isn't something that being a physician has a monopoly on. The "I want to help people" shows that you haven't really thought the decision through, and adcomms want people who are 100% sure that this is what they want (well at least as sure as you can be before you start doing it).
Well, I want to help people physically and medically. A nurse sure does just that, but s/he doesn't have total control of what s/he can do, a doctor does. Does that make me sound like a control/power obsessor? is that bad?
And so you've always dreamed of it . . . for the longest time I dreamed of being a ballerina, seriously, this was my lifes aspiration for about 14 years. I thought it was beautiful artform and was joyous when dancing, but when it came down to it and I learned more about the proffession I realized it just wasn't the best fit for me. I learned about a dark side of the world of ballet that was really against everything I believe in. I learned my career would be over by my 30s. I realized that I had a mind was capable of lot of other careers that might be more fulfilling for me.
Did you incorporate this part in your answer? This is all about you and your life, and nothing about helping others. Is this acceptable too? I am asking because I don't want the committee to misunderstand my ultimate reason for pursuing medicine.
The point is that just because you always dreamed it doesn't mean its really the best fit for you. You have to have more specific things that attract you to it to prove to an adcom that you've learned about the proffession and thought this thru a bit. There are many people who think they want to be a doctor, and then when they see the proffession for what it is, they realize they don't. The adcom asks this question to make sure they aren't admitting people who will later regret their decision.