Why We Drive on Whatever Side of the Road We Drive On

Or at least here are some possible reasons as to how our common (but sometimes differing) practices evolved. My question is, how far do people carry this? For example, we drive on the right, so I tend to walk on the right side of the sidewalk and push my grocery cart down the right side of the aisle. I'm always a little off-kilter when I'm walking a shared bicycle/pedestrian path and walkers are directed to walk on the left side, even though I understand the rationale (narrow path, bicycles move much faster than walkers, and walkers can see the cyclists coming). But not all paths suggest that solution, so most of us walk on the right, and cyclists get used to calling out, "On your left," as they pass us, so that we'll know not to step over into their path and risk a collision.