What makes something rude?

Watch the full video here: • Is it rude for Autisti...
So, what makes something rude? I mean that genuinely. I’ve been trying to figure this out for decades.
Is it saying something that hurts another? I think it’s more clear when someone is hurtful or mean to another. That’s easy to call rude, and we’ll get to hurtful behavior in a bit. But there are lots of things that are considered rude that aren’t actually hurtful.
Is it violating a social norm? Any social norm?
What if the norm is something like, spending time with family is socially more rewarded than spending time alone? Then I want to ask myself why? What is it about the two that, as a society, we value one more than the other?