Why can't I have everything I want?

We're in Target today and Mwende is walking "next" to the cart. It becomes an endless stream of "Mama, I want this!" "No Mwende, put it back.", over and over again. I told her that she can't have everything she wants. She then asks one of life's greatest questions. "Why can't I have everything I want?" Tried to explain how we couldn't afford it, that it wasn't good for her anyway, how no one on earth really gets everything they want. It didn't sink in.