I remember an American who came to our school once telling us that they had to repeat some sort of 'America is awesome' thing (I pledge allegiance? I can't remember, it's been a while) at school every day since he was young.
So basically indoctrination, that's why Americans are so patriotic. It's got nothing to do with whether the country is great or not, it's that the children are taught about how awesome America is before they can even talk properly. It's kind of like how Stalin or Kim Jong Il got people to think they were awesome, byt forming a cult of personality, only instead of a person, america do it to a country.
On a related and hilarious note (for me) he also told us that in his textbooks and lessons he wasn't taught that America lost the Vietnam War. When he started doing history at GCSE (we did Vietnam) he was genuinely shocked to find out that they hadn't done so good there. And this was someone who lived in New York and went to school there, not some backwoods Louisianna swamp kid.
EDIT: Sorry, should also point out how it's not really 'true' patriotism, it's more like nationalism. That whole jingoistic 'my country tis of thee' stuff that rednecks and Republicans spout isn't patriotism. Captain America patriotism is very different 'I am loyal to nothing, except the American dream.' Not cultural boundaries, not nationalist pride, but the dream of a free country where people are judged on the merits of their character and their actions instead of the luck of their birth or their choice of religion or partner or anything else.
Nationalism is 'my country, right or wrong.'
Patriotism is 'My country right or wrong, if right to be kept right, if wrong to be set right.'
So many so-called patriots forget the second half of that sentence.