Some countries we drive on the left; some countries we drive on the right. What is the origin of each style?
Originally, all the riders were on the left because that's the side you mount the horse from, and you mount the horse from the side of the road. You don't mount a horse standing in the middle of the road.
This changed around 1700 when advancing bridle technology and improved wagons with metal axles, metal suspensions, bearings and fittings made it possible to drive large teams of horses and heavier loads. The driver still mounted from the left, but then he would drive the wagon on the right because when you pass another wagon you got to be able to make sure you are not going to hit him, and since you are riding on the left side of the team, you want the other wagon to pass you on that side so you can see him.
These large teams were primarily found in France and Germany, so a tendency developed in those countries for teamsters to drive on the right. Then, when the French Revolution occurred, the switch to the right became set in stone because up until then peasants and teamsters went towards the right, and the nobles rode in the center, the left, as was customary. After the Revolution, the center became verboten territory and everybody rode on the right edge, even the single horseman, lest they be mistaken for a stray nobleman. When Napoleon marched across Europe, he spread this practice in his wake. In America, they followed the old custom like England, but then switched to the French way during the American Revolution due to their new affinity with France in the war against England.
Thus, there was a division between old and new. Old being represented by England, Netherlands, Portugal, etc. and new represented by America and countries influenced by the French: Germany, Italy, Russia, Poland, etc. Over time the continental countries gradually switched to match France/Germany and only England, the Netherlands and their colonies remained as left-handers. Japan was a left-hander too, because they followed the single-horse pattern. So, as a general rule only England, the Netherlands, their colonies, and Japan drive on the left. Everybody else drives on the right.
With regards to Japan, I should also mention that the saya, the scabbard of a Samurai's katana, is always on the left and therefore a Samurai always walks on the left, because if he walks on the right his saya could touch that of another Samurai, a very very serious offense.
I've heard that the left hand rule is so that sword-bearing riders can engage each other, while the right hand rule is due to men driving large carts being able to use whips on their oxen without striking oncoming carts.
Take that as you will.