The British have always been lefties. The first forms of driving, dating back to antiquity, were that of chariot or horse-drawn cart. In Roman times, carts kept to the left. This tradition was carried through to the Middle Ages, as left-sided driving was much more practical in a violent feudal society; an attack on the road was best taken from the right as the driver would be better positioned to quickly pull out his sword. This practice was common throughout medieval Europe, until it was changed by the French. During the French Revolution in a bid to turn absolutely everything on its antithesis, the driving laws were changed to "keep right." Napoleon quickly adopted the right-hand rule as law for two reasons--a display of absolute power in even the minutest of details, and the singular fact that the emperor was left-handed-- according to Ed Wright's "A Left Handed History of the World."
The World Wars were a milestone in English--French diplomacy, as historically the two nations congenital rivals. During Napoleon's reign, the right-hand rule was forced upon the conquered lands of the empire, if for anything other than to oppose the French, the British kept to the left in defiance of the Napoleonic Empire. Other countries to follow suite were Portugal (though it switched to the right in 1920) and the Austro-Hungarian Empire. Today, road side is still a distinguishing factor of pride between the British and French.
Ultimately the driving on the "opposite side" of the road was a form of resistance, a resistance not unlike that of a child who has been told not to do something by his mother yet does it none the less for the reaction. Those who opposed Napoleon opposed his driving rules, thus to build a greater arsenal of "lefties" or "righties," the driving rules were spread to possessed colonies. The United States did not adopt driving on the right until the Revolution as a form of resistance to British ways. According to James Joyner on outsidethebeltway.com, the same pattern is shown throughout colonial world history--defy the colonists by defying their laws.
Actual standardized law of road directions did not occur in the United States until the early 20th century with the popularization of Ford cars. Ford's mass-produced vehicles were made with the controls on the left side of the car, therefore making the car suitable only for right-hand driving. As early Ford controlled the market, the right-hand driving rule became standard. Much of the world adopted the right-hand standard--much of the world exclusive of Britain, Australia, India and a few of Britain's other former colonies.
Some speculate that right-side driving is representative of Christianity as the right side of the body is the purer side (though the heart is on the left) and therefore Christian nations drive on the right. This explanation holds very little water, however, for Great Britain is traditionally Christian. There's no basis for implying that Australians are morally more sinister than Canadians because they drive left.
