Why do people say, "Thats just great!" whenever something bad happens?

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This is one of my biggest pet peeves. Whenever something goes wrong people say, "That's just great!" And, of course they don't actually mean the bad event or misfortune is a good thing. And, the worst part is, sometimes I catch myself using that phrase as well. And, there are other times when I'll say something I don't really mean. Like when my child breaks a vase or stuffs toilet paper down the sink, instead of making a big deal over it, I just say, "It'll be OK!" Do you find yourself doing that too? I wish I could just say the truth, but sometimes the truth is too harsh. And, I guess that's why I do it. So, why do you think people say the opposite of what they mean when something unfortunate happens? Do you think it's just sarcasm? Or, maybe it's their way of shedding the best light on a bad situation? Are there other times when you say the opposite of what you really want to say?