This article suggests multiple reasons behind US opposition of Nord Stream 2 project:
- Europe's energy independence
- Ukraine loss as less gas would transit it
- monitoring technology that could also be used for espionage
- Countering America’s Adversaries Through Sanctions Act:
a clause requiring the administration to “prioritize the export of United States energy resources in order to create American jobs, help United States allies and partners, and strengthen United States foreign policy.” To German officials, U.S. attempts to halt Nord Stream 2 are part of an effort to boost U.S. liquefied natural gas exports, which accounted for 5 percent of Europe’s LNG imports in 2017.
This article argues about political leverage that might be incurred by this project:
The United States and some other German allies have bristled at the project, warning that it could give Moscow greater leverage over Western Europe.
As a side note, the main criticism (along with arguments against each one) is mentioned on the project site itself.
Because it involves Russia. The priority of the US is to keep Russia and the EU divided (divide et impera!) in political and economic terms.
Especially economic: the EU and Russia are natural partners. Russia needs European finished goods and technology. On the other hand, the EU needs the natural resources in which Russia is so rich.
Such a partnership would give a boost to both economies. The US obviously doesn't want this.
The main reasons are certainly political, i.e., reduce potential Russian leverage over Europe and starve Russia of export revenues. These reasons have already been mentioned, even if they might deserve a more detailed explanation.
Apart from these security-policy arguments, there is also a purely economic reason, which hasn't been mentioned yet: The US have newly become a net exporter of oil and gas and want Europe to buy American liquefied natural gas (LNG) (see Europe is fast-becoming a natural gas battleground for Russia and the US, CNBC, Jan. 2019, ).
However, liquified natural gas would be more expensive than pipelined Russian gas:
This is the reason why the US government has been conducting a hysterical campaign for years against the expansion of the Russian Nord Stream Baltic Sea pipeline. From next year, up to 110 billion cubic meters of cheap pipeline gas can flow through the two pipes. This volume will not only make Germany an important distribution point for natural gas in Europe.
As Harald Hecking and Florian Weiser already calculated in 2017, Nord Stream 2 can lower gas prices in Europe and worldwide to such an extent that the LNG business is not even worthwhile with state subsidies. At his meeting with Donald Trump, Russian President Vladimir Putin had therefore offered to coordinate prices so that "even the most expensive suppliers" could survive.
For orientation: In 2017, the average German import price was USD 5.62 per million BTU, while in Japan, for example, USD 8.10 was paid for the same unit of LNG. In the Baltic states and Poland, of all countries, which already import LNG on a large scale, gas prices have risen again in the past two years.
Translation of a part of a German heise.de article with www.DeepL.com/Translator
To sell our own LNG to Germany. Really. We may cheery-pick a selection of moral arguments against Nord Stream 2 and keep silent about moral arguments in favor of Nord Stream 2, whichever the all these arguments happen to be, but really, we simply make LNG sales and have made Germany more dependent on us.
This makes European countries too much economically dependent from Russia. I do not think it should be lots of other reasons.
Russia is large, politically unstable country with nuclear weapons and a history of unfriendly relations with USA (the Soviet times). Nobody knows where it will turn. This seems sufficient to see expansion of the influence of this country as not preferable.