Back to aviation - why do aeroplane captains sit on the left, and whirlybird ones on the right?
Concerning helicopters....
Firstly, the pilot doesn't sit on the right in all helicopters; some are flown from the left, some from the middle. It is true that most are flown from the right. I've heard several explanations, but the most accepted one seems to be that the first helicopter pilots were taught by fixed-wing pilots, who sat on the left, so the student sat on the right, learned to fly from there, and continued to do so.
When flying solo it's sometimes a weight and balance thing; for example, the R22 can't be flown solo from the left.