The words are invariably a signal that you are about to be dumped. I remember every single time this has happened to me, it's indelibly etched into my memory.
It's all about rejection, I think. The only other time that I've experienced the queasy, sinking feeling that goes with these words is on the single occasion when I was fired.
Even when you're expecting it, such a situation is always deeply shocking and viscerally disturbing. The fight or flight reaction that inciteful mentions is immediately invoked, and you get that "butterflies in your stomach" feeling. You want to do something, anything to stop what's coming next, but you can't. You're frozen, like an animal caught in the headlight of a car.
Rejection like that leaves a wound, no matter how expected it was. Sometimes, it takes years to get over it - but you never lose the terror you feel at those words.