Not trolling. Serious question.

I have been riding for a while and never gave any though to whether I passed on the right or left. I was all conditional, both were ok and no announcements were needed. This is esp true if we are talking about intense rides or training races.

I have read, in several threads over the last few days, of people complaining about riders passing on the right.

Can someone please explain this to me. Is it a regional thing? Has it come about because of some traffic laws or some other outside reason?

Esplain please.