The US is trying to grab it; they're actively threatening with sanctions to dozens companies that are building a new gas pipeline at the moment (https://translate.google.com/translate?sl=nl&tl=en&u=https%3...), and they're successful in that some companies have backed out. The pipeline is already mostly finished though. But basically the US is trying to stop it because it'll reduce the dependency of Europe on American resources.
Or you could think of it as the US helping (some of) its NATO allies in Europe. Poland, Estonia, Latvia, and Lithuania have been vocally against Nord Stream 2. Just because Western Europe is for it doesn't mean all the other countries in Europe are as well. Even the previous president of the European Council came out against it.
