The reason for driving on the left is quite logical. In feudal societies, where most people were right handed, you held your sword in your right hand and your scabbard was on the left. This meant that when approaching someone on horseback you would have your sword hand closest to the person coming towards you in case you needed to defend yourself, and your scabbard with your sword in it safely further away on your left hip.
The question is then, why traffic in so many countries then changed to the right side. I am sure Google knows. I must look it up one day.
Interestingly, Sweden used to drive on the left side of the road and changed overnight in 1967 – without a single accident! I am sure the transition was facilitated by the fact that Swedish drivers always sat on the left side of the car, unlike other left-driving countries where the driver sits on the right. In Stockholm the underground and commuter trains still run on the left.