Came across this interesting story.
History and origin
About 35% of the world population drives on the left, and the countries that do are mostly old British colonies. This strange quirk perplexes the rest of the world; but there is a perfectly good reason. Right-handed knights preferred to keep to the left in order to have their right arm nearer to an opponent.
In the past, almost everybody travelled on the left side of the road because that was the most sensible option for feudal, violent societies. Since most people are right-handed, swordsmen preferred to keep to the left in order to have their right arm nearer to an opponent and their scabbard further from him. Moreover, it reduced the chance of the scabbard (worn on the left) hitting other people.
Furthermore, a right-handed person finds it easier to mount a horse from the left side of the horse, and it would be very difficult to do otherwise if wearing a sword (which would be worn on the left). It is safer to mount and dismount towards the side of the road, rather than in the middle of traffic, so if one mounts on the left, then the horse should be ridden on the left side of the road.
In the late 1700s, however, teamsters in France and the United States began hauling farm products in big wagons pulled by several pairs of horses. These wagons had no driver’s seat; instead the driver sat on the left rear horse, so he could keep his right arm free to lash the team. Since he was sitting on the left, he naturally wanted everybody to pass on the left so he could look down and make sure he kept clear of the oncoming wagon’s wheels. Therefore he kept to the right side of the road.
It goes on to talk about other countries driving habits with this one interesting story:
Pakistan also considered changing to the right in the 1960s, but ultimately decided not to do it. The main argument against the shift was that camel trains often drove through the night while their drivers were dozing. The difficulty in teaching old camels new tricks was decisive in forcing Pakistan to reject the change.