Canadians, Kiwis, Aussies and their flags

Why do they insist on putting their flags onto their backpacks and whatnot?
I'm an American who's done quite a bit of traveling in Europe in the last
couple years and haven't experienced any problems with people disliking or
being rude to me. So why do they feel the need to proclaim that they're not
American? Like you wouldn't be able to tell from their accents! It's kind of
insulting to me too, because they don't want to be mistaken for an American,
like there's something wrong with being American.