It's how our perception works. You look at the picture from left to right. So the direction of move or objects on the picture indicates the meaning of composition.
Move from right to left indicates for example struggle and move from left to right looks easier, lighter.
Since I'm more movie guy so example from war movies. "Our" (or good) army attacks always from left to right (check Private Ryan for example).
There are tons of such details which can change the meaning of picture or movie scene.