Because we've all grown up in a society that from day one has latched onto the idiot self esteem movement and tried to convince us that we're the greatest thing ever.
So when someone implies we aren't, it conflicts with everything we've ever been taught to believe. Almost our entire culture is inundated with false messages: Just be yourself, everyone is equal, you can be whatever you want, etc. Reality is tough to deal with if you've never had to deal with it before, and most Americans live such a sheltered existence that very few have.
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SmartMuffin - Because anything less would be uncivilized
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