Rules of the Road. Which places drive on the left?
"In most early motor vehicles the driving seat was in the middle. Later some carmakers chose to have it nearest the centre of the road to help drivers look out for oncoming traffic, whilst others chose to put the seat on the other side so that the drivers could avoid damaging their vehicles on walls, hedges, roadside gutters and other obstacles. Eventually the former idea prevailed." — Wikipedia
Anyone who’s wondered what you do when the side of driving changes, watch for signs. :)