First of all it's not just Germany, this pipeline includes many other EU countries like the Netherlands.
Second we already buy it from Russia, but have to get it through the Ukraine, which is a huge risk for the EU.
The Ukraine is extremely corrupt, even more so than Russia and you need to bribe everyone to not get fucked over as they are as poor as the Philippines.

Russia is corrupt, but they depend on the gas sales, so they are reliable and stable.
The Ukraine is not and a huge security risk.

Nordstream 2 literally is a risk reduction measure, anyone who is against it either directly profits from the current pipeline or is being ignorant of the situation.
In case of the US, they simply want to sell their own expensive gas to Europe.