Is it not obvious? Since WW2 we have underwritten European social welfare states by subsidizing defense. We secure sea lanes and choke points (Suez, Hormuz). Which protects global trade. We provide cheap financing to Europe and access to our deep liquid markets. We give you geopolitical insulation from Russia and china. In short you enjoy peace, trade and welfare at a heavily subsidized cost due to American strength.

And of course US does this out of kindness of its heart. Like everything in the world. Of course it is not its own choice and will and it definitely gets nothing in return.