If you are raised in a language in which you are taught to read left-to-right, top-to-bottom, it's how your brain subconsciously pays more attention to what's to their left first rather than what's to their right first, if not instructed to do so another way (at least for most people).
It's why they say, when designing a powerpoint slide, it's important to place the biggest points on a slide in either the top-left or the bottom-right (bottom-right, since, when reading a page of a book, we also train our brains to pay attention to the very end of the page when skimming through pages).
So, when meant for an American audience, it's most natural to place the person we should be focusing on to the left when it's a photo-op with just one other leader. It just looks "right" when lining a couple people up.
Fascinating. Would a press conference in, say, Japan, do it the other way because they read in the other direction?
