As the hencq said, most if not all European nations had their share of bad times due to nationalism and in consequence the idea of nations got deemphasized. That is the origin of the European Union, together instead of against each other. This admittedly works much better in good times and a we against the rest mentality quickly springs back when the outlook worsens.
For many Europeans the American attitude towards the country and the flag looks a bit like a fetish. It's a piece of the Earth within a man-made line on a map, a piece of fabric designed by some guys. The important things are of course what this symbols stand for, freedom, the American dream, going to the Moon, helping to end World War II. But then again this is also only half of the truth or do you also think of slavery, racism or the millions of victims in US (supported) wars when you look at an US flag?
Similarly the obsession with the constitution looks a bit strange from here. »I can have guns, it's good to have guns, it's in the constitution!« It is just a piece of paper, a set of laws. Some guys thought that those would make good rules to run a society and so they wrote them down. But maybe they were wrong in some regards, a constitution is no eternal God-given truth.
It really makes a difference where you grow up and to what ideas you are exposed. For the better or the worse.