I grew up in country whose neighbour country speeks the same language, but has roughly 10 times the population.
What people in big countries do not realize is just how much their internal affairs radiate outwards. And this creates an atmosphere of:
small: "I know everything about you"
big: "I don't even know who you are"
When the big one, now on top of overlooking you
actively gives you shit for not being a good neighbour (despite feeling to have been at the short end of the stick for decades), you will have a rally around the flag effect of gargantuan proportions.
I have learned over the years that the US is a huge, diverse and multi-layered phenomenom, so it deserves some nuance. But as an European I feel Trump and his minions giving us shit for e.g. US car manufacturers doing nothing to try to appeal to the European market (except for rebranding asian cars and whatever the fuck the PT cruiser was meant to be) is really a bit rich.
Most Europeans always felt their political leadership relied too much on the US, not because we extracted value from that, but because it was comfy to not have to think for yourself. They even tried to paper over the abuse of trust that came to light in the Snowden revalations (How would the US have reacted if the Germans had hacked the phone of the POTUS?).
Now with an openly hostile and erratic US president (EU offered a 0-tarrif deal to Trump before he introduced "retaliatory" ones) people feel less inclined to talk good about an already asymmetrical relationship.
Trump doesn't want partners, he wants vassal states. And if you really want those you either have to (A) use miltary force or (B) give them some psychological fig leaf they can use to convince themselves they are not indeed vassals.
Trump took the open secret of the fig leaf away and mocked Europe for accepting it in the first place. And the European population feels their trust is breached.
I don't say I like it to be that way or that the feelings are 100% justified, but that is the cause of the sentiment.