Disagree. European countries are not "very very different in all aspects". There is some variety, but Europe is locally continuous: differences between, say, Poles and Czechs are not that pronounced and you can find people that are hard to classify as either of these nations, the same could be said about differences between Belgians and Dutch (or Belgians and French), Norwegians and Swedish, Ukrainians and Russians, Portuguese and Spanish, Poles and Belarussians etc etc. Looking at Europe from bird's eye view, one may notice a few large clusters (Slavic nations, Germanic nations, Romance nations) that differ somewhat, but they blend into each other (migration, intermarriage) making people roughly compatible.
Note that doesn't mean that people of similar culture do not hate/despise/fight each other. Vice versa, it seems that the most fierce rivalry happens precisely between parties that are similar to each other (see Balkans, or Polish-Russian rivalry, or Norwegian-Swedish relationships, or Portugese-Spanish, historical English-French, etc etc), just like relatives happen to quarrel more often than strangers.