Why are European countries so close to each other, but they develop completely different cultures?

I wonder whether the cultures are "completely different". Again, a bit more specific question would help to understand the thought better.

Neighboring cultures influence each other and I think it's easy to find that there are cultural regions within Europe, where cultures share many features with their neighbors.
The most familiar to me are the Nordic countries, or this Northern region in the whole. Every country has its own culture, but they resemble each other. The further you go, the bigger the difference, but it's a gradual change.

For example: what's the big cultural difference between Finland and Sweden, except the language? Or between Finland and Estonia? They are different, because they are different sovereign nations, but the difference is like between relatives.
It's harder to say about other regions, but I imagine that for example the Mediterranean cultures resemble each other in many ways, or the Central European cultures.

Linus Skov talked about the birth of the national cultures and that's true: the cultural areas were even smaller in history and current nations have been built by unifying the smaller nations or tribes under one identity and culture. The borders are often arbitrary and many (or all) countries have significant minorities from their neighbor countries. They have cultural influence in every country.

What exactly has determined the sizes of the current states, that I cannot answer. There are unique reasons in every country, I think.
Finland has formed from those Finnic tribes that were under the Swedish rule. Those tribes didn't share Finnish identity, it only developed through the centuries. Originally there were just the Finns, the Tavastians and the Karelians. The Savonian and the Ostrobothnian tribes actually developed during the Middle Age.
There were other Finnic tribes in the East, like Karelians and Vepsians. They went under the Novgorod'r rule and the developed to their own nations. There were centuries of war to and fro and Finland's border moved to the East and to the West. Lapland was so far in the North that any kingdom didn't have power to conquer it in the Middle Age. Lapland was finally divided for different states in the 1800s.
There is a large Swedish speaking minority in Finland - and even bigger Finnish speaking minority in Sweden.

That's what happened here. The areas and the states formed according to the strength of the states and stabilized during centuries and when the states lost their military power to go conquering their neighbors.

This is an always interesting video. Many must have seen it, but I think it's still worth sharing. Thousand years of European borders in three minutes:

Regional differences are not all that uncommon in human societies however I think that the vast differences can be attributed to history and how people have been divided and united by the ruling class of kingdoms or nation states.

Nation states and kingdoms create, artificially and naturally, a hegemony amongst populations under their dominion. They usually impose a single language, single currency, and a single set of laws, and a single cultural identity on it's people. Rulers would also unite these nation states with armies, law, culture, religion, and also make war with other nations, dividing the international working class. They would draw imaginary divides between people even if they looked the same, spoke a similar language, or had a similar culture.

European nations are so culturally different because unlike America they developed from warring kingdoms, and later nation-states that became culturally, economically, and politically unified. After the feudal era the strongest kingdoms consolidated their power through the destruction of all other kingdoms and the unification of these kingdoms under a common banner, language, and economy.

In America there was no warring kingdoms. To the contrary, we (the white man) went over and instituted our own hegemony over the entire landscape. It had started as a largely unified nation and whilst there are still large regional differences, particularly North and South which lead to a civil war, the nation-state that was the USA created a national hegemony.

Europe at this point actually isn't all that diverse. It used to be almost every valley in Germany and France had their own language and culture and regional identities existed all over the continent, but gradually, each country has kind of adopted one identity per country.

In olden days, you might, for example, consider yourself from Jutland, Funen, North-Zeeland, etc., but today, you're from Denmark.

Anyway, I think this question is probably based on a comparison between the Americas, which are considerably less culturally diverse. This is because the cultures currently there, didn't evolve "naturally," so to speak. If the Native Americans had developed there in peace, most certainly we'd see a wide variety of cultures, but right now, they're mostly a reflection of whatever countries had colonies there.

If you look at other continents, like Asia or, the most diverse continent of all, Africa, I think you'll find that there is just as much if not more diversity than in Europe.

Hope my answer helps

Because groups of people when separated and out of communication with each other develop separately - even more so in the pre-literate age.

To take two cultures at the opposite end of the world - the Maoris of New Zealand and the Moriori of the Chatham islands. The Moriori were Maoris who moved from New Zealand to the Chatham islands in the 16th century and lost touch with the Maoris for 300.

They developed quite separately - the Maoris became extremely war-like, while the Moriori developed as peaceful people. The Maoris invaded them, killed nearly all of them and ate some in the 1830s - 1850s.

Now as to European countries - they developed separately as well, due to geography, language, religion, history.

So the English were a pre-Celtic people related to Basques who were invaded by Celts, then Saxons, Danes, Normans etc. and became a people distinct from the French or Spanish or Danes etc.

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