EU countries are much more uniform than the rest of the world in their level of welfare support and social development (e.g. UK and even Portugal are much more alike than say Bangladesh and the US).
Besides, there has been a lot of movement from poorer countries to richer countries within the EU, and many people in the richer countries are angry about this. See Brexit etc.
There has also been a lot of movement back towards such countries as soon as their wealth levels improved (or UK ones got worse), which happened in no small part thanks to remittance payments. It's the sort of movement you also observe in-country when industries in certain regions do better than others. How many people from Scotland moved to Southern England in the last 100 years?
But that is the whole point of the European Union, which is also expressed in things like the infrastructure and development funds: we promote solidarity and cooperation between neighboring countries, rather than competition that will ultimately result in war.
It's a shame how you only need a couple of generations to forget everything about their history.
Or why are migrants, against the laws of the EU, streaming through and past Hungary, Austria, Germany, Denmark, in an attempt to claim asylum in specific countries, when each country that they're literally walking through is also amazing and beautiful?
The guy that you replied to is effectively describing what's happening in the EU.
What stops this from happening in the Schengen Area, where some countries participating have welfare states and some (I believe) do not? Genuine question - I've heard other people make the same argument before and it seems like we have a pretty good real-world example to either verify or refute it.