I have been told that the side you drive on depends on whether your nation signed up to British or French alignment.
Apparently, the whole world rode on the left for reasons of self defence/greeting as Old Farmer states. However, Napoleon decided that it would give him a tactical advantage if his troops rode/drove on the right (don't ask me the logic in that). Therefore countries that historically drive on the right can trace their roots back to French occupation, those historically on the left to British.
