I spent 5 years in the USA and what I loved the most was to read US history. Living in DC, I joined a bunch of think-tanks and saw how vibrant the political life is. And I was an admirer of the USA because of its amazing history, just as you say.
I just think the USA can do better than it currently is, and closer to what it is capable of being. I think it lost its ideals and drifted from the honour of a republic to the tragedy of an empire. The UK of course has the legacy of a more cruel empire, and no country in the world is free of blemishes. (And the Arab world is a basket case making its own recovery.)
But I just felt so sad when comparing what I saw Americans do - amazing people, such a pleasure to be around - and what their government did in their name - locally and internationally.