The "requirement" for [fixed wing] captains to sit on the left may have been derived from the rule that aircraft flying along a line feature such as a railway line must keep to the right - that in turn seems to have been derived from the nautical rule that ships heading in opposite directions should pass port side to port side. If the captain is following such a line feature he needs to be able to see it, so sitting on the left should give a better view.
Bear in mind that most commercial pilots will have done most of their flying training sitting on the left, so it should not be as unnatural as it may seem to use the left hand. Indeed, there is an argument that moving to the right hand seat may need more adjustment but - as others have implied - by the time someone gets to be a first officer, using either hand is not a great issue.