In aviation the explanation is quite simple and for once does not have a nautical source.
When navigating from point A to Point B, if the weather was bad, it was common practice to follow a line feature, a road, a railway, a canal etc. and this did lead to some regrettable 'head on' accidents. It then became a convention that when following a line feature it would be kept on the left, that way opposite direction traffic would be separated as they would now be flying down opposite sides of the same line feature. As soon as aircraft grew to side by side crew, pilots/engineers and later pilots/pilots, (more commonly known as pilots/co-pilots), then the captain sat on the left so that he could accurately follow the line feature, he was still 'the pilot'.
Helicopters do their own thing, some are flown from the right seat, (most, I think), whilst others are flown from the left seat and I have no idea why.