As a child and young teen I had a very vivid imagination, setting stage upon stage in my mind (my Barbies and I were very close). As an older teen, those kinds of internal plays got a little more dangerous. It became easy to fall into the fantasy of my own mind where everything ended up perfectly, everything exactly as I wanted it. Then real life came, knocked on my forehead, and reminded me that plays are nice, but they always end. Unfortunately, part of my growing up included booting the players from the stage, and once you do that, they don't really want to come back (no matter how helpful it would be as a writer to have the little people in your head act out plot lines a la Robert Louis Stevenson...).
Post Script: "Robert Lewis Stevenson, famed spinner of dark tales, had his own little men in his head, that he exploited for fame and profit." Hear more about that here: Radio Lab Fascinating stuff.