I sleep on what would be the right side if standing at the foot of the bed, but I always consider the left, since I look at the left/right paradigm when I'm actually lying in the bed.
Anyway, it has always been this way as long as I can remember and likely started because when we got married the alarm clock and phone (when there was only landlines) were on the left (facing the bed) of the bed so that was her side. We have just naturally kept to that arrangement, even though our individual stuff next to our sides of the bed could conceivably be on either side.
It also remained that way due to in our current home there being a clearer and more direct path to the bathroom on her side as she has always availed herself to nightly visits more than me (especially during pregnancy), though that has admittedly been less of the case since I (and my bladder) have aged
.
There has also been little incentive to change over the years due to me consciencely and unconsciously spooning her during the night and her preferring to sleep on her right side, which also works out for me due to favoring my right side as well due to left shoulder pain if I lay on it too long.
In fact, the few times we have slept on opposite sides of the bed for whatever reason it felt strange and alien. Oddly enough, this is only the case in our own bed, as when we are away from home, while we mostly keep the same arrangement, but have been known to switch sides with no problem, so the dynamic differences are kind of weird.