know-it-all

'A number of times, Romney chastised his opponents for not following the “rules” of the debate. That may work well in high school forensics, but in real life no one likes the kid who comes across as the know-it-all. Remember, Romney’s biggest hurdle is not to convince voters he’s the smartest guy in the room, it’s to convince them that he’s real.'
http://news.yahoo.com/analysis-rick-perry-takes-off-gloves-las-vegas-031156581.html

Two questions:

1) I am not able to grasp the exact meaning of in real life no one likes the kid who comes across as the know-it-all. I think there is some America culture context involved here. Could any one tell me why people don't like such a kid in America?

<< Second question should have its own thread. >>

The Americans call such a person a 'know-it-all', but the British say know-all. Here is the definition:
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You can see that such a person might be unpleasant. The point is not that he knows a lot, but that he behaves as if he knows more about everything than anyone else. In fact, he may not know much at all. In particular, a kid who thinks he knows everything probably doesn't.

<< No longer necessary. >>