General Zod said:
who in many ways live under our protective wing
I think it is that kind of attitude which leads to a lot of criticism; whether justified or otherwise, the perceived superiority/police-the-world foreign policy is going to piss a lot of people off, and result in increased criticism. That results in a vicous circle, that ends with the defensiveness JK mentions, and more criticism.
I also feel it's something to do with the age of America as a country; being so young, a lot of the vociferous population is still very patriotic, which can make criticism hard to take. A lot of European countries are old, and many people aren't so patriotic. As such when criticism is levelled at us, most will agree if they think it is true, rather than being irked.
I was thinking about this as I have a couple of American friends who are both anti-Bush, but feel compelled to defend him when the subject comes up in conversation, because they think an attack on the current government is an attack on the country. If anyone from the US criticises Tony Blair here, virtually everyone will heartily agree with them.
BTW, those are just observations, and are not intended as criticisms. Both attitudes clearly have pros and cons, it's not intended to make you defensive