I was born and grew up close to the french border, in a region that was contested between France and Germany and often changed hands. I was lucky to be born after the french-german friendship started to take roots, but I'm well aware that my country and France used to be arch enemies and fought three devastating wars between 1870 and 1970, not to count the petty little border conflicts. I still remember the day that you could just cross the border without a passport check and I'm still amazed every time I cross that border, that the border post, the guards, everything, just disappeared and that the border is now essentially a sign just like I'd enter a city saying "Welcome to
France". People that were born elsewhere or later than me don't even recognize the spot any more where the border was.
To the contrast, my partner was born and raised close to the Belarussian border and that's still like I remember borders: Armed guards, passport check, fences, need a visa to cross. 20km of queue, 36hours of wait for trucks trying to pass the border.
_This_ is what the European Union achieved. It's still messy and there's a lot of things that go wrong and possibly that will never really change, because a union of such diverse people with such diverse and distinct history will never be able to figure out how to cater to all needs in a fair way, but the achievement of having no separating border between so many countries with different rules and regulations, it's just amazing and I cherish it every day and I'm deeply saddened every time I meet people that can't see it.