Why do so many British people hate Americans?

I see all kind of things all over yahoo answers from British people talking about how America screwed England over in WWII, how we are all so stupid. Just a lot of ridiculous insults in general. These people seem pretty arrogant, and hypocritical. But being an American, most people I know don't have any sort of grudge against England. So why the hatred? And is it common in England to despise the United States?