The reason why we're all screwed eventually

It is all to do with staircases. Especially spiral ones. Everyone knows that spiral staircases are nifty if you need to protect yourself from invading knights, who typically enter the staircase from below after invading your castle. With most people being right-handed, you obviously build the staircases so that when going up, the staircase forces the left hand of the person to be on the outside. This is convenient as this increases the difficulty of the attacker to hit you with their sword while at the same time making this easier for you to do.

We no longer stab people with swords, because this is considered not nice. However, we still have spiral staircases. General convention is also in all civilized countries to go on the right side of the road. This also fits well, because in spiral staircases which have been built in the proper fashion, by sticking to the right side of the steps you're doing things right. Right? Right. And that's because the probability of taking a misstep is higher when going down and often the outcome of a misstep is also a lot worse on average when going down. To mitigate this bit, it's only logical to allow the people coming down to occupy the wider bit of the step. And by sticking to the right side of the step of a properly built spiral staircase, this happens. It's all very logical and nice, and when people adhere to this emergent rule of how to act properly, everything goes smoothly, people don't get killed by stumbling in the stairs and people don't need to stab each other with swords.

However, this is not often the case, alas, as some people cannot understand the system dynamics and why certain things are better done in one way and not in another. This means that you will have deviants who don't act in the proper way in spiral staircases and this initially causes confusion, annoyance, and ultimately more accidents. And all because they are selfish and cannot optimize the system. It is those people who will ultimately cause fragile but optimal systems to break apart.

Lesson of the story? When you go up a spiral staircase, you step on the narrower bit of the steps. When your going down, you step on the wider bit. If you don't do this, I will stab you with a sword. Capisce?