From https://www.usnews.com/news/best-countries/articles/2022-02-24/explainer-why-did-russia-invade-ukraine

“I think NATO, and the invitation for Ukraine to join NATO at some point in the future, is simply just a pretext to potentially invade Ukraine,” he says, referring to Russia. “Ukraine is not a member of NATO, it doesn't have any of the NATO guarantees, and so there is no hint that Ukraine will become a member of NATO soon.”

Putin, specifically, does not want Ukraine to join NATO “not because he has some principled disagreement related to the rule of law or something, it's because he has a might makes right model,” adds Bradley Bowman, the senior director of the Center on Military and Political Power at the Foundation for Defense of Democracies, a non-partisan research institute focused on national security and foreign policy.

“He believes, ‘Hey, Ukraine, I'm more powerful than you. And because I'm more powerful than you, Ukraine, I can tell you what to do and with whom to associate,’” Bowman says.

Beyond the concern around NATO and other demands related to weapons and transparency, Russia’s nature of expansion is also at play when it comes to Ukraine. Some Russians, Putin included, feel Russia has a claim to the former Soviet state.