Thats assuming that dreaming is totally in your head. So you could say that your not aware enough of the Dream realm for it to be stable. But this like most things belong in another topic.

I think what your saying applies to most games. When the player is no longer in a certain area, then the guards, scenerios and everything else in that area cease to be. Until you go back into that area. THis is mostly because no system is powerful enough to simultaniously keep everything running in real time. But I think your referring to the equavalent in dreams. When your not looking at something, your mind sees no purpose of keeping it intact. so it fades away. And then, when you look back, it creates it again, not entirely the same. Thus the instability of dreams. If your not looking behind you, then there is no reason for your brain to create anything behind you. but if your focusing on something then your mind also focus's on that object/place/person.

At least, thats the way I see it, provided dreaming is totally in your head that is....