Or common courtesy / "culture" rules. That said, people on the spectrum have trouble with phrases like "What's up" or "How are ya"; a quick answer like "Not much, you?" is in a lot of cases a lie so it doesn't come out as smoothly as cultural norms seem to expect, and a genuine answer is not something the asker expected, especially if it's about e.g. work or hobbies that the asker doesn't know much about, or if you have to assess your current mood / situation to describe how you are.