Because NATO could put missile systems in Ukraine that could deliver a payload into Moscow in a matter of minutes before Russia has time to respond. It also cuts Russia off from the Black Sea. It's arguably more threatening to Russia's security than missiles in Cuba were to American security in the Cold War.

Putin's actions are immoral and illegal, and I'm absolutely against this war. But even given that, the fact is that for years, the West has been living in its own world where it thought it never had to seriously consider the security concerns of a nuclear-armed great power, and now reality is slapping them in the face. The West has consistently refused to do actual diplomacy with Russia, and thus bears a great deal of responsibility for this current crisis.

Edit: Gonna add that what I'm saying isn't even some kind of out there, Tankie viewpoint (although it's basically marginalized as such now). Goerge Kennan, Pat Buchanan, and Henry Kissinger all thought NATO expansion was dangerously provocative of Russia, and argued as much back in the 90s. I remember following this back in my lib days in 2014 and listening to Dan Carlin's Common Sense, where he said that playing these power games in Ukraine and promising them NATO membership was going to lead to disaster, and that we needed diplomacy with Russia.