Why do some countries drive on one side of the road (like the US) and some drive on the other (like the UK)?

It had to do with Knights, Chariots, Caravan and Carriages. During the feudal time the Knights would pass on the Left side of the road in order to hold the reins with their left hand and defend themselves with the sword on the right.

In the late 1700s, the shift from left to right that took place in countries such as the United States was based on teamsters' use of large freight wagons pulled by several pairs of horses. The wagons had no driver’s seat, so a postilion sat on the left rear horse and held his whip in his right hand. Seated on the left, the driver preferred that other wagons pass him on the left so that he could be sure to keep clear of the wheels of oncoming wagons. He did that by driving on the right side of the road.

During the time when the Horses were used to pull the Carriages the driver had to be situated on the right side of the carriage so that they won't hit their passengers on the rear when they use their whips.

After the Carriages were no longer in use and cars have become the mode of transportation for most of the people around the world. 34% of the world's people live in left-hand traffic countries and 66% in right-hand traffic countries. Most of the countries which retained driving on the left side of the road were island or isolated from other neighboring countries.

Keep in mind after the the war of independence it was declared that the USA was as far/Different from the British Empire as possible. And lets not forget that when we drive a horse drown cage we like to hang the light on the left side and now we find our self driving on the right!

Because everyone else is wrong !