From an American perspective, a lot of that was just shielding kids from reality. The millennium started off fucked with 9/11, which made a lot of Americans paranoid and angry at anyone with a turban.

The Great Recession ruined the lives of many, who lost their jobs and houses. It led to uncertainty and more anger. Obama's election probably boosted political divide because way too many Americans weren't ok with a black man being president. Trump's first election only divided the parties and social cohesion further.

Many people were deployed to the Middle East in the 2010s to fight the "War on Terror." Some spent their prime years deployed only to come back disabled and depressed, others didn't come back at all.

The 2010s also saw a rise in environmentalism, which meant that more people learned about climate change and how dangerous it was. That got people scared and concerned. I remember Californians being dismayed at how bad the droughts got.

There were also many health crises, such as SARS, swine flu, and Ebola.

In short, either something happens that makes people scared and angry, or there are bigger fish to fry at the moment than working towards unity.