Everyone thinks that they're right...unless they're left. I've driven on both sides of the roadway as a matter of national policy, driving etiquette and survival. The problem comes when you've been driving on one side of the road and you change locations (like driving from Hong Kong into Mainland China or driving from Thailand into Laos or flying from the UK to almost anywhere else). It still works, until you turn a corner and instinctively revert to the side of the road that you've been driving on up to that point.
In Hong Kong, it matters a great deal. When you change to Mainland China it matters less because the rules of the road are "fuzzy". In Scotland on Hogmanay (New Years) people drive on both sides of the road without much knowing -- or caring.