So I'm not really qualified to answer the entirety of this question, my background is in German studies and political science so although I'm familiar with the larger scheme of European history I don't feel comfortable speaking to all of Europe and the United States. Instead, I'll focus on the development of German and European nationalism in the late 19th/early 20th centuries as background for the discouragement of those ideas.

So to start, it's important that we understand what we're actually discussing here with regards to nationalism and patriotism. Your wording, "near-nationalistic patriotism," seems to be coming concepts of the terms as they're colloquially defined, especially in the US - that nationalism is a hyped up version of patriotism, rather than a distinct term. There's certainly a link between the two, but as Lukacs (2005) puts it,

patriotism is linked to the land, to a particular country, while nationalism is connected to the myth of a people, indeed to a majority; patriotism is traditionalist, nationalism is populist.

Blank and Schmidt (2003) also provide some useful distinctions (and a lot of additional reading if you're interested) - nationalism is characterized by a feeling of national superiority, uncritical acceptance of national, state, and political authorities, and the suppression of ambivalent attitudes toward the nation, while patriotism allows for both love of country, as well as criticism of national authorities, questioning of the nation's aims, and negative nation-related emotions. At least for this comment that's the framework I'll be working from as I move on to actually answering as much of your question as I can.

So, Germany. In the modern day, Germany tends to be uncomfortable with concepts of nationalism, to the point of many Germans being uncomfortable with even displaying the German flag. This discomfort comes primarily from the country's experiences with nationalism in the 19th and 20th centuries. It's important to remember than Germany as we know it is a relatively young country. Prior to the unification in 1871, there was no unified German state. There was no collective history of the German nation, there were instead many separate histories of various German states (Mommsen, 1984). After 1871, there was a Germany, but there was yet to be "Germans" - necessitating the nascent German state to construct a sense of national unity out of the fractured identities in the state (Confino, 1993). The national bureaucracy was an arm of this task, with the creation of agencies such as the Committee for Nature Conservation and Heimat Protection to instill love of Heimat (roughly, homeland) into the German nation (Confino, 1993). It's worth mentioning here that this process was not necessarily unique to Germany. While I can't speak to the specifics in other countries, the general trend is the same - fomenting of nationalism via state apparatuses. Massimo d'Azeglio, prime minister of the Kingdom of Sardinia said of Italian unification, "We had made Italy: now we must make Italians." So there's a general trend as Europe left the 19th century and headed towards WWI of state-level encouragement of nationalism. I won't go much into detail on that, I'm sure this sub has plenty of discussion on nationalism and WWI, but it's useful background as we get to why this trend reversed itself to the situation we have now in Europe, where nationalism is largely discouraged.

The short version is that nationalism destroyed Europe. The Axis powers were, as extensions of their fascist ideologies, hyper-nationalists. Fascism was (and is) the extension and culmination of standard nationalism (Smith, 1979). Under the Nazis, the idea of the German Reich and the German Volk were espoused as inseparable. The ultimate goal was the idealization of the nation, at the expense of those outside it. This is the nationalism that Blank and Schmidt describe taken to its furthest extreme. The defeat of the Axis powers was, as a result, the ultimate defeat of those nationalist ideologies in the 20th century. Peter O'Brien puts it better than I can:

It was National Socialism, paradoxically, which discredited German nationalism. Defeat in, and division after the war made the unity of nation and state a practical impossibility. On a deeper ethical level, the base deeds of the Nazis laid bare the moral depravity inherent in nationalist thinking.

The history of modern Germany (and much of Europe) has been periods of fervent nationalism which led to the downfall of the states which espoused that ideology. So for the modern German nation, living with the legacy of WWII and the Holocaust, the idea of nationalism has become inherently distasteful to most. It's worth mentioning that groups like Pegida and AfD run counter to that trend, but the majority of Germans find it hard to reconcile their historical experience with anything resembling nationalism. Patriotism, being the love of their land coupled with skepticism and criticism of the negatives of the state, has become the norm.

I would like to talk a bit about the US, but keep in mind this is my opinion as an American, not a fully formed response. In contrast with Germany, the United States won both World Wars, becoming a super power in the process. The Cold War led to decades of tension, fomenting patriotism, and oftentimes nationalism in the US. The US has, for most of its history, gone not been crippled the way Europe was after the World Wars. To put it bluntly, we haven't been taught (as a nation), for better or worse, the dangers of nationalism.

Sources:

Blank, Thomas, and Peter Schmidt. "National Identity in a United Germany: Nationalism or Patriotism? An Empirical Test With Representative Data." Political Psychology. Blackwell Publishing Inc, 03 June 2003

Confino, Alon. "The Nation as a Local Metaphor." History and Memory 5.1 (1993): 42-86. Indiana University Press.

Lukacs, John. "Patriotism versus Nationalism." The Chesterton Review. N.p., 01 Apr. 2005.

Mommsen, Hans. "History and National Identity: The Case of Germany," German Studies Review 7 (1984): 5

O'Brien, Peter. Beyond the Swastika. New York: Routledge, 1996. Print.

This doesn’t address the entirety of your question, but part of the answer has to do with the distinction between civic nationalism and ethnic nationalism. Ethnic nationalism implies that nations are rooted in shared bloodlines and a common culture. This view of the nation is derived the Romantic-era belief that each people has a specific spirit and destiny – the Germans use the term volksgeist - that distinguishes it from other peoples.

By contrast, Civic nationalism, or liberal nationalism, assumes that the nation is based on a common citizenship, and is expressed through shared institutions (such as Congress), and political principles such as democracy and the rule of law.

The late 19th century was the period in which the modern nation state was forged, and during this period many European countries leaned toward an ethnic definition of nationhood, while the United States tended to espouse a more civic form of nationalism.

u/50ShadesofBray has discussed how this played out in Europe, so I’ll focus more on the United States.

The Civil War was in many ways the crucible through which modern American nationalism was forged, and the rhetoric surrounding this identity tended to be based on political principles and notions of citizenship rather than any claims to ethnic unity.

So, for example, Abraham Lincoln’s first inaugural address was almost entirely devoted to a reading of the Constitution that held that the secession of Southern states was a violation of its principles:

I hold that in contemplation of universal law and of the Constitution the Union of these States is perpetual. Perpetuity is implied, if not expressed, in the fundamental law of all national governments. It is safe to assert that no government proper ever had a provision in its organic law for its own termination. Continue to execute all the express provisions of our National Constitution, and the Union will endure forever, it being impossible to destroy it except by some action not provided for in the instrument itself.

He returned to this theme again with the Gettysburg address. Unlike his first inaugural, which tended to imagine the country as a union of states, Lincoln now clearly spoke of the US as a “nation” – specifically, one “conceived in liberty and dedicated to the proposition that all men are created equal,” and one that required the sacrifice of fallen soldiers “that the nation might live”, with the hope that “this nation, under God, shall have a new birth of freedom and that government of the people, by the people, for the people, shall not perish from the earth.”

In other words, Lincoln framed American nationality on the basis of a commitment to equal rights and democracy, rather than any shared ethnicity or culture.

He underlined this theme again in his [second inaugural address] (http://www.bartleby.com/124/pres32.html) in 1865, framing the war not only as a struggle for political principles, but explicitly attacking a regime of racial slavery in which one group of people would be denied rights on the basis of their ethnic background, stating “we shall suppose that American slavery is one of those offenses which, in the providence of God, must needs come, but which, having continued through His appointed time, He now wills to remove.”

In many ways, the modern-day United States has inherited this civic conception of nationalism, which is less inclined toward xenophobia and the exclusion of minorities than ethnic nationalism. So for this reason, the flag-waving patriotism you observe in the ROTC and elsewhere in the US might be considered more innocuous than its counterpart in Europe.

I'd like to add on to the discussion here by pointing out that what you point out as "the rest of the world" is actually just mostly Europe. Reddit's userbase is mainly English speaking countries and Europe, so when you ask "Why is it that Americans are more patriotic than the rest of the world on Reddit?" you are comparing America to other industrialized, predominantly white countries. For reasons very well explained already, nationalism has had a profound effect on Europe.

But nationalism still runs strong in some parts of the world less affected by the destructive nationalism of the Nazis. Ethnic nationalism is still very strong in South Korea and Japan, and is considered an essential component of their citizenship laws (immigration to these countries is extremely low with most citizenship applications falling under spousal sponsorship).

Finally, I'd like to point out one more interesting facet of American nationalism: the marriage of the military to patriotism. Your post completely fuses the idea of supporting the military to supporting your country -- which is common in America. Other countries have a much more distant relationship with the military; you can be patriotic without supporting the military, but in America it's hard to separate the two. I'm not knowlegeable enough about the link here but I thought it was an interesting observation that you might want to consider!

I think it has a lot to do with the way the American experience in World War II differed from that of most other nations.

Before World War II, nationalistic pride, often bordering on and sometimes spilling over into virulent, militaristic, racist nationalism, was pretty common all over the world. Look at Italy under Mussolini, obviously Nazi Germany, the Japanese Empire, the USSR, and so on. Pretty much every major actor in World War II was motivated by ideology that we would today find abhorrent.

The result of this war, as everyone knows, was absolute destruction for the losers, and significant destruction for the victors. The only major actor which could be said to have truly benefited from the war was the United States, which emerged from the war as unequivocally the most powerful country in the world, in large part because, while many soldiers died, the homeland was undamaged.

I mean, consider the fact that both Pearl Harbor and September 11th (violating the rule momentarily, but it's part of my overall argument), have had massive impact on the American psyche, while compared to the apocalyptic devastation Japan, Germany, and the USSR suffered during World War II, they are relatively minor events.

Basically what I'm trying to say is, I think nationalism has been working well for the United States for a long time, since the unification of the colonies and the expansion west. The country hasn't experienced the kind of horrific trauma as a backlash of militarism that many other countries experienced, so it continues onward.

I think there's a flaw in your premise. Many, many countries around the world play host to quite a bit of nationalistic rhetoric. In Denmark, the example you raised, 21% of the legislature is currently controlled by the Danish People's Party, a right-wing party which has made itself a bit of a reputation for populist, anti-immigration politics and its case for a return to traditional Danish values. Judging by the backlash Trump gets for his comments on immigrants, the kind of rhetoric espoused by the DPP wouldn't receive much of a warm welcome in the United States! And virtually every other country in Europe also has its own relatively powerful political party(ies) with a platform based on pure ethnic nationalism, including the UK, France, the Netherlands, etc., including notably Greece. You may remember reading a few years ago about the connections between Greece's Golden Dawn party and neo-Nazism following their big electoral win. Of course, this is all outside the premise of this subreddit, but I believe it addresses the present-day European aspect of the question.

Casting our net a bit wider and moving back 20 years, there are plenty more cases worth looking at. China implemented an education reform known as "patriotic education" beginning in 1992. Key points include emphasis on China's mistreatment by the hands of foreign countries and the enduring unity of the Chinese people through those times of adversity, and a simultaneous downplaying of civil strife, as compared to earlier education initiatives which emphasized civil conflict as a main driver of China's historical progress. Patriotic education has recently been a very hot topic of research as the first generations to go through it come into maturity, and Pew research polls indicate that China's national ethos is becoming gradually stronger and more united as the country continues to develop. Of course, this development has not been isolated, and another reply has already mentioned how Korean and Japanese national identities are every bit as much (when not more) based on foreign conflict, eternal unity and exceptionalism, as are the national identities of most countries across East, Southeast and South Asia. And they often add a racial component, too.

Also in the early 1990s, the Yugoslav Wars were reaching their peak in Southeast Europe. There are many ways to read the Yugoslav Wars, which saw what was once one relatively prosperous country rent into seven, but no matter which way you approach it, rising tides of ethnic nationalism played a crucial role in mobilizing and motivating people. Since separation, six of these seven new countries promoted a powerful new national identity through the creation of new national languages (in many cases a newly defined language altogether,) new ethnonyms, new national heroes and icons, and new "official" approaches to history and the arts which emphasize the nation's unity throughout the ages. So kids who've gone through school since independence have been through a lot of classes that all emphasize this independent identity as paramount to the survival of the nation. And today, across the Balkan region, these nationalistic narratives absolutely dominate both politics as well as popular discourse of national issues to this day. Of course, not everybody shares the same perspectives, but discourse is nationally and ethnically charged to a level not seen in the US.

Really though, lots of countries all over the world embrace nationalism unashamedly. Take a look a Turkey, which likely wouldn't exist today if not for the emergence of a powerful strain of Turkish ethnic nationalism following World War I. It's a case study of how modern nationalism can bring a country's people together. The Turkish national movement managed to dramatically simplify the Ottoman formula by cutting out all of its complex traditions and titles, and all of its divisive controversies. In the face of hostile foreign intervention, they managed to bind together a language, traditional way of life, common history and broadly common experience with a flag, a secular ideology, and an army. It was successful far beyond the expectation of most observers at the time. And this is the formula followed today by most nationalist parties around the world. And while Turkey has slowly allowed Islam and Islamism to reenter the public sphere, any such moves faced heavy resistance from mainstream political parties and the army well into the 1990s, underscoring the centrality of these institutions to the Turkish nation-state.

So it's not really fair to say nationalism is purely an American phenomenon today. But perhaps many Americans do display a little more reverence for the symbols of the state than citizens of other countries. I think this plays right into the difference between ethnic nationalism and civic nationalism that some others have already described. For Americans, those symbols of the state--the flag, the troops, the monumental government architecture in Washington--are the only symbols that pull everybody together. So they're the natural outlet for people who want to display their patriotism. And maybe a case could be made that Americans have a lot more of this kind of civic pride as compared to other countries.