The story I've heard about which side of the road to drive goes back to Napoleon's time. The majority of people are right-handed, so when horse-riding soldiers using swords to fight with rode towards each other, they went right-side to right side. Napoleon trained whole squadrons of soldiers to fight left-handed. As they rode against the British, they came up on the opposite side to their sword hands and wiped them out.
After that, the French decided to ride on the right and it carried over into the motor vehicle age. Since Europe is all connected through land borders, the other countries followed the French style.