I believe that the reason we fly left hand circuits as a default, unless told otherwise, due to the old rotary engines of the first world war, and before, era.
As already said the propellor was bolted directly to the crankcase and there was a huge spinning mass on the front of the aeroplane. Due to the direction of rotation, a left turn caused the nose to pitch up (good thing) and right turn caused it to pitch down (bad thing).
Incidentally I believe this is why the captain sits it the left hand seat, as he would have had the airfield on his side for a convential circuit.