I think it varies case by case. Sorry, I don’t believe in a equal and just relationship between the biggest economy in the world and economies that are few order of magnitude smaller. As well as I don’t believe in a relationship between 40 years old man and 17 years old girl.
But let’s imagine that this is a win-win relationship, say between Estonia and NATO. I understand that Estonia is getting from them, but what USA is getting?
What did USA gain from joining the allies in Europe during WW1 and WW2? Besides the moral and humanitarian reasons, which I actually believe played a big role even though I'm not American and I know it's easy to be cynical online and disregard those, they also gained a larger market for their products. US hegemony has benefitted the US for decades. Same thing applies now, the more countries that are friendly and politically aligned the better. And NATO isn't just the US, the Baltic airspace is currently being protected by Portugal and Romania.
You claim that the economy of EU+UK is "a few order of magnitude smaller" than US?
USA is getting a strong partnership next to one of its worst enemies.
Let me pose this question: If US would drop out of NATO, would that mean all NATO countries would drop out, or would the rest of NATO still form an alliance?
But let’s imagine that this is a win-win relationship, say between Estonia and NATO. I understand that Estonia is getting from them, but what USA is getting?