Always Something Else

A few weeks ago, I was making a routine Wal-mart run when I saw something that made me stop and wonder. I was in the back, by one of the pay-phones they have outside of the restrooms. As I stood there, waiting for my companion, a little girl skipped away from her family and walked up to a pay-phone. She pretended to put money in, and proceeded to play-talk into the phone as if she were a thirty year-old woman speaking in self-assured confidence.
It made me think: why is it that when we're children, we always think about being grown-up? We play house, we play school, we play work...we dress up in our parents' clothes. I think one of the number-one things children talk about (or at least are asked) sometimes is "What am I gonna be when I grow up?" It's all about growing up.
BUT,
then the child grows. And the child finds himself or herself wanting to be a child again.
Why is that?
It seems to me we're missing something by always wanting to revert or progress, and never just to be.
Why is it that we always want what will be or what was, but never what is? Well, I suppose it would defeat the purpose to want what is--why would you want to want what you already have?