Carriages..cars, most of which had the driver on the left...had the aircraft guys putting the pilots on the left...simple....hence also the advent of left hand patterns...
Helos... the ascending blade of a helo creates more lift in forward flight then the ascending blade...so when going super fast...the descending blade can't really keep up and a yawing rolling tendency occurs..the weight of the pilot on the same side as the forward moving/ascending blade helps counter act this...
Anyone who actually flies helos single pilot on x countries will tire pretty quick of sitting on the descending blade side.... pushing the cyclic toward the side producing the most lift all day long...