The European Union is not threatening the United States in any way. The United States is not facing a threat from the European Union. That does not mean that the EU could not pose a problem from the United States down the road. Relations need to be kept up in order to remain friendly.
Europe isn't strong enough as a federal government to stand up to the United States. The USA is strong because there isn't as much infighting among many regional groups that gets in the way of economic progress. Europe is still too divided among ethnic groups and different political parties to stand up to the economic might of America.
The United States faces a lot of threats from a combination of lunatics and countries that are rightfully pissed off at it, but the European Union isn't on anybody's radar as a perceived problem. Granted, relations in the region aren't stellar, but they far exceed how things stand in many other parts of the world. Not at the top of the concern list.
The European Union was a bold experiment that seems to be slowly failing, and I think it will eventually fade from prominence. There's no fiscal incentive for economically prosperous nations such as Germany to remain in a union with welfare states like Greece, who cannot pay its own way. The sense of nationalism among the individual nations is too great to be overcome to make it a viable long-term union.
The United States is not facing a threat from the European Union. Just because the nation is getting bigger does not mean that they will soon dominate anything. The United States should not have to fear anything from other nations because we are allies with their country. We also provide a tremendous amount of support to them and always will.