What do you want?

When I was little, I wanted to become a scientist. I grew up a little, I wanted to be an nuclear engineer(like work on nuclear fission and stuff). After that some time later, I had a rough going and I got knee deep into songwriting. I don't know how. But I liked it. And a little later, I thought "maybe I should sing my own songs"  because who could sing the lyrics I wrote better than myself? And now I am, doing none of the above mentioned things but still hoping I'll make them true someday! 
Why does this happen? Why can't you just DECIDE and cut through. Why do you have to be pushy and go the way life flows and not the way you want it to? 
I don't have answers. I'm still figuring it out.