But all other countries bordering Russia are supposed to be fine with having a heavily armed country next to them?
It's really quite a dilemma. It's easy for us to dismiss the Russian fears of being invaded by the West. It's hardly something that makes sense to us. But looking at it from the Russian perspective, they feel justified in being paranoid. The US was involved in ousting a Russian-friendly government in Ukraine less than a decade ago. The US has consistently used Russia as an object of ridicule and something to be suspicious of, most recently as 2016 when the Democrats aroused fear by falsely tying the Russian government to efforts to elect Trump.
We look at these events as less threatening than Russians do, because we have a different, and more rational perspective. But it's worthwhile to understand that our actions are perceived as highly threatening, and we seem to be uninterested in developing any sort of trust with Russia.
We mistrust them and they mistrust us. It's a self-fulfilling prophesy. Unfortunately, they are acting on their fear by attacking a sovereign nation. And even then, the current US government is using that event for it's political benefit by blaming this war on their own domestic failures. The US government also appears content to let the war continue.
Without question, Russia's actions are horrendous, and has resulted in needless loss of life. But the US and other western governments are using this war to accomplish other policy goals.