It's really quite-simple. Trump has put on the table a promise to stop them from stealing from the United States.
Yes, stealing.
First, in national defense. Europe steals something around $100 billion in national defense spending that they ought to be spending every year, but we end up covering.
Trump has made clear he intends to put a stop to that crap whether it's Mexico by stopping their citizens, including their drug dealers, from invading the United States or other nations that enjoy our personnel and weaponry protecting their land and people instead of them expending their own money and personnel.
But second, and far more importantly, is found in medical care and drugs which Europe uses to make their "socialized" schemes workable. The national defense theft is tiny by comparison; Europe's theft in this regard, along with the rest of the world, is close to $800 billion annually. That utterly dwarfs the defense spending rip-off; we don't even spend that much in total on national defense in any given year!
Trump is hated for his policies all right -- he's rabidly against those nations stealing from the United States, and justly so, exactly as he is against China stealing from our manufacturing sector.
This is why Europe hates Trump -- he proposes to stop their wholesale theft.
And while he'd have to get Congress to go along with the national defense side when it comes to the health care theft he can stop it in the Executive by merely enforcing existing law against all health-related businesses in the United States.
PS: Between national defense and health care that's roughly a trillion dollars a year President Trump would take out of the "expense" side of the Federal Budget, or roughly a quarter of the total immediately and permanently. The economy would explode upward were this to take place, as would employment and real purchasing power for everyone in America.