Most theories try to explain this constellation of forces relative to the education, more precisely the habits of literacy. (This should therefore indicate that the eastern cultures are wielding an inverted set of meanings for left and right, which hasn't quite been believably proven at all.)

It is my belief that differentiation between these sides is founded on some physiological, or at least biological reasons. No need to call upon the division of human brain to rational and intuitive quite yet. Still, I can clearly recall some impressions from as far as early childhood that have witnessed to nicely defined attributes of two sides. The memory of even the first and most innocent reactions to the concepts of sides always remembers the right as something dark, solid and earthly, and the left as light, spiritual, and so much closer that the distance was always more possible with the right, which could then be - handled. A bit of encouragement to this thinking could be found in the linguistic observation of the words for right - always sinesthetically dark and solid sounding, while the words for left have the opposite flavor - light and airy, impossible to catch, like a spirit itself.

As far as attention to the literacy is concerned, I'd rather try to argue that westerners therefore write "from inside out" and easterners " from the outside inwards" - and this could be found in accord with the differences between western and eastern thought itself. Seemingly, these contrary orientations are bound to introduce a nightmare of ambivalent inputs. However, the opposite information is interpreted by equally opposite system (as long as images stay within the area of origin...), so the end result is the same - mostly agreeing on differentiation between the directions of inward and outward. So, even though a very "inductive" (as opposed to "deductive" of the west) character, or the particular compositional organization can lead the currents inside the frame to flow from right to left, a westerner's eye will still, in the vast majority of practical cases "read an image from left to right" - recognize a constant flow of forces in that direction. That means that every object aiming left will look as if resisting something, while the one aiming right will let itself go with the flow of the whole. Frequent example can be seen in the image of the leaning square (pict. a) and b)): the square leaning right appears to be "blown away".

Possibly the best example we can found in car ads: to figure out the car company's attitude, no need to go further than the direction the vehicle is pointed. Almost all airlines' logos have planes going toward the right, since nobody is silly enough to fly "upwind". And probably the most famous is question of the side Madonna holds the little Jesus on. Picture definitely unfolds from left to right - and this doesn't necessarily indicate the path of our eye. This old-time favorite composition analysis tool is just an unexistent materialization of the general structure of  image, construction built by the all forces of the composition.

Of course, the influence of "intuitive" and "rational" side of image doesn't stop at determining the general flow direction. There is a very efficient mapping at stake here, down to the nuances of every point in between of the two extremes, the left and right edge. Mapping sometimes so pronounced, that we can pull the line down the center, clear as a border between night and day.

Just as we could check the effect of up - down orientation by turning the image (and so the all of our accompanying illustrations) upside down, we can also view the same pictures using the mirror, or perhaps thru the paper against the light. And all this so that it will never again be the same weather we want to flop an image just for the sake of the layout, or not.
in the frame:
left and right

Although massive, material and square - the concrete block on left is perceived rather intimately, and all its weight just as our support. That way, closeness to the viewer is achieved, an identification with the presented space, regardless of it having an undesirable overtone.