That's because since WWII, Europe has been coddled by the US govt and the money and protection it provides through stealing money from it's citizens to cover the bill. Now Trump is threatening that and people are losing their minds, lol.
Also because the US as one of the winner in WWII designed Europe to be dependent on the US after the war.
And it's time to end all that shit, lol. So for that, I do have the upmost respect for what Trump is doing there. Sure, it's bad in the short term for Europe because the fun ride is over, but like you said, it forces Europe to become self-sufficient, which is a good thing in the long-term. Because if money is going to be stolen from my pocket through taxation, I damn well want that money spent here, not abroad.