Russia -- by which I mean Putin specifically, as the average Russian man or woman on the street doesn't care -- is mad about the loss of much of the formerly Soviet sphere of influence in Eastern Europe.
That's it. There's simply no military threat to Russia from NATO. NATO has existed since 1949, and has never once attacked the USSR or Russia, nor would it. Why? One word answer: Nukes.
Beyond that, NATO is constructed as a defensive alliance. Article 5 has never once been deployed outside of the Afghanistan War, an actual attack on real U.S. soil.
Of note, Russia still maintains significant, one might even say controlling, influence in countries like Belarus, Serbia, Syria, and several Central Asian states. Of course, that's not enough for an imperialist like Putin. Never is.
Russia could have regained a lot of what it lost through soft power: by Being Nice.
The 2000's was a great opportunity to do so. Putin was still young, fresh off of a (mostly) fair election victory, hadn't yet changed the Russian constitution to rig things in his favor for life, and presided over a Russia that for once had some hope for its future. Money was pouring into Russia from around the world. Putin was one of the leading international critics of the Bush Administration's disastrous invasion of Iraq. Rightly so. Putin Had A Point. Perceptions of the U.S. in Europe were at a low point.
However, Putin's subsequent pivot to hard-authoritarianism squandered all that goodwill.
And now, post-Ukraine invasion, it's impossible for any Eastern European state today to look at Russia and see anything other than the old hammer-and-sickle. Even Lukashenko's pathetic public comments lately acknowledge that reality.
And so, darn near all of Europe, and the vast majority of Eastern Europe (only exception: Hungary) has landed all-in on support for Ukraine. Sweden and Finland applied to join NATO. As always, better to fight them Over There than Over Here.