Really it is more a matter of expression and reading - they're still there but not expressed in the same way nor your cues and conventions picked up necessarily. Just because their tone may not convey it doesn't mean they have deadened emotions - that is depression.

Think of it like a cat vs a dog wagging a tail. Usually a cat with a wagging tail is agitated - a dog with one is happy. Just because a cat with a wagging tail swats at you doesn't mean they are happy to attack - it means they are stressed out and you disregarded their clear signal!

I say usually because there are outlier cases that learn the other conventions. See Tally the Husky for an example who even does the "cat-loafing" pose of hiding all legs while sitting.