Because if you’ve never lived in a situation where you had to constantly be monitoring another person, making sure you weren’t sitting in the wrong spot, or walking around too much, or breathing too audibly, or giving off the wrong vibe, in order to try to avoid an explosion of rage aimed at you, you can’t understand why ‘walking on eggshells’ is exactly the right image. None of those are joke examples, by the way — the rules are set up for you to fail. You try to balance in the exact quiet center, and you know that you can’t do it forever.