If you ever have to say 'lighten up' to someone, you've failed twice. The first time, when you misjudged an interaction and the other person reacted in a way you're unhappy with, and the second time, when you issue this instruction, one that is guaranteed to evoke precisely the opposite reaction you're intending.
I'll add "I was joking," to this list, because it's an incredibly lame excuse for a failed interaction.
One more: Raising your voice while you say, "You're just going to have to calm down!" (And I'll add librarians yelling at kids to be quiet...)
It's completely valid to come to the conclusion that someone else can't be a worthy audience, conversation partner or otherwise interact with you. You can quietly say to yourself, "this guy is a stiff, I'm never going to be able to please him." But the minute you throw back instructions designed to 'cure' the other person, I fear you're going to get precisely the opposite of what you were hoping for.
(Generally speaking, the word "oh" is so neutral, it's a helpful go to pause while you wait for things to calm down.)