Cultural encroachment (think monroe doctrine), nato encroachment (think Cuban missile crisis). Less important but probably considered: geopolitically Russia needs buffer states in the east because there's no natural defenses/borders. It's an easy invasion from that direction.
Key quote: “Russia is always anxious about foreign penetration – not only in terms of military involvement and political engagement but also in cultural terms,” Fasola told FRANCE 24. He pointed to the so-called “colour revolutions” that brought pro-Western governments to power in Georgia (2003) and Ukraine (2004) – and which the Kremlin perceived as “instruments of the West to drive those countries away from Russia”.
Also Ukraine has massive coal reserves (that's where the name Donbas comes from) and some of the richest agricultural land in the world.
This assumes that Russia tries to capture and hold ground though and we've seen how insurgency wars have worked out for the Russians and Americans for the last 3 decades.
Good article: https://www.france24.com/en/europe/20220222-moldova-then-geo...
Key quote: “Russia is always anxious about foreign penetration – not only in terms of military involvement and political engagement but also in cultural terms,” Fasola told FRANCE 24. He pointed to the so-called “colour revolutions” that brought pro-Western governments to power in Georgia (2003) and Ukraine (2004) – and which the Kremlin perceived as “instruments of the West to drive those countries away from Russia”.