1. Decorum. Because a hubbub or a brouhaha would be unseemly.
2. Because You’re British. It’s what’s expected of you. It’s what comes naturally. Your reaction to an unforeseen and potentially frightening event should be one of unflappable fortitude and apparent indifference. No flapping. Got that?
3. Consequences. If you should flap then someone with a plummy voice will be forced to administer a swift slap and command you to “pull yourself together”. That constitutes a scene. No one wants a scene.
4. Tradition. It’s what that formidable lady, Great Aunt Harriet, would have wanted. She kept calm, probably in a winceyette nightgown. I doubt there was much carrying on though.
5. Because It’s Relevant. It’s an iconic slogan that’s rooted deep in the past – though it was never actually used during World War II – but its message still holds good today. Why worry about that double-dip recession that you can’t do much to influence? Why worry about potential terrorist activity that you can’t stop? Why worry about the man peering over your shoulder as you read this? No, on second thoughts, do worry about him. He’s probably up to no good.
6. Because The French Do Neither. We keep calm and carry on. The French do not: They je ne sais quoi. On bicycles, probably.
7. Because You Are Told To. Your poster demands it of you; your greetings card demands it of you; your t-shirt demands it of you; your mug demands it of you; the coaster on which your mug sits demands it of you; your key ring demands it of you; your fridge magnet demands it of you; your shopping bag demands it of you; your deckchair demands it of you; your cuff-links demand it of you; your sticking-plasters demand it of you; your tea-towel demands it of you; your chocolate bar demands it of you; your flight-bag demands it of you; your apron demands it of you; your rug demands it of you. It’s bloody everywhere. Seriously, if one more person buys me anything else that says “Keep Calm And Carry On” on it, I won’t be held responsible for my actions. I will freak out and stop.
Taken from here.