Woudl it be less offensive if instead of saying "lighten up" they said "it's no big deal. Why are you always so up tight anyway?"

Certainly the latter is not a demand and is even a question, but I would argue that even barring that, it's far more of a problem.

Of course that's offensive. But you're still doing what knightenvy was not, and going beyond "I don't understand." It's perfectly fine to say you don't understand something. But from there, you should try to gain an understanding, and your quote (like the 2 previous definitions of "lighten up") don't do that. And yours adds victim blaming, which antiterra's doesn't really.

But the point is that both of these are really ways of just shutting down conversation, right? "Lighten up" isn't a demand. It's just a way of cutting off the conversation through a retort.

Yes, that too. Man, it's almost as if 'lighten up' is a complex phrase with many different aspects of meaning :) I like the way you do think through all of these different meanings, though