This unspoken set of rules can turn an otherwise rational person into a flailing, helpless victim in a sea of self-perpetuated social anxiety.
It's like we're all competing in a game that no one wants to play. And even though you can't ever win the game, you can prevent yourself from losing by pretending that you like playing long enough to be allowed to stop playing.
There is a special kind of awkwardness between two people who don't know each other well enough to interact effectively, but are familiar enough that ignoring each other's presence isn't really an option. No matter how much you like the person, you dread encountering them because you only know two things about each other and once you've covered those two things, there is nothing else and that is terrifying because you aren't good at ending conversations and that makes the horrible, strained silence all but inevitable.
But eventually you do run into one of these acquaintances and even though you both sense the impending awkwardness and desperately wish to avoid it, you have a social obligation to say hello. So you do, and the conversation derails even more quickly than you expected.