Society appears to present us with fixed rules that we must follow. Some are immutable laws that will never change. Time will move forward and gravity will pull us down to Earth. Others are governing laws—if you steal you will be punished.
Finally, there are customs. Unlike gravity, these customs can be influenced, generally by those with the most to gain. This can take years or even decades and eventually it feels like it has always been this way. Which is how we arrive at rules such as engagement rings are deserved and must have a large, sparkly diamond, preferably a carat or more, and cost a lot of money.