Why driving on the left? or on the right?

That is ze question.
I always wondered the reason why different countries would have a different system on the road.
Apart from the usual frog/rosbeef jokes no one ever could tell me.
Thank to the instructor in the bicycle mechanic course I undertook this month,
I now know and thanks to internet I now can tell and share.
In this article, it explains why do the British drive on the left?

in brief this is why:
"In the Middle Ages you kept to the left for the simple reason that you never knew who you'd meet on the road in those days. You wanted to make sure that a stranger passed on the right so you could go for your sword in case he proved unfriendly."

In France Napoleon's successful attack strategies and his massive ego are responsible for France and its colonies to have kept the driving on right system.