Something that's been bugging me

I've only noticed this recently.. actually no, I've always known it but it's only started to bug me recently. Why do most people think that their country is the BEST country in the world and can do no wrong and every other country is crap? And if you dare say anything bad about their country (or city) they get very very defensive. I guess there's nothing wrong with being proud of where you live or grew up but still... I just don't understand..