The free movement of people and goods is a fundamental idea of the European Union, and one that’s driven tremendous economic benefit to the region (not to mention the individual benefit from unburdening travel). It’s embodied in the Schengen Agreement and in other agreements.
When you enter a European country you go through passport control. If you connect at an airport to elsewhere in Europe you’re generally taking what amounts to a domestic flight. Travel within Europe, for the 400 million people inside the Schengen Area, is like travel inside the U.S.
Sweden has now imposed border controls with Denmark and Denmark with Germany.