Aside from habit alone, such as how scrollbars are customarily at the right, this phenomenon may result from psychological or even neurological factors. Psychologically, the side of your dominant hand is where you prefer to keep things you see as dangerous or more difficult to control. This is why a male and female couple usually walk with the male at the right. The male's job is to protect the couple from outside in…
This is damn interesting and fascinating. Wonder to how many other user cases it can be connected to.