The obvious reason is punishing Russia for Ukraine invasion. But, there are other non-obvious reasons, namely,
- Russia is increasingly becoming again a military, political and economic competitive after the collapse of the USSR (Soviet Union). Ukraine was a part of the Soviet Union.
- Rise of Russia has been historically considered a multifaceted threat to Europe.
- Ukraine has the potential to be a nuclear state, having inherited the know-how and technology of the Soviet Union. Russia wants prevent Ukraine from becoming a nuclear power as its neighbor, on the top of being a NATO and EU member.
- Russia considers Ukraine as part of greater Russia.
- Russia feels threatened by EU and NATO expansion towards Russia and breach of earlier commitments by USA and EU not to do that.
- Mistreatment of ethnic Russians by the current Ukrainian leadership in Kiev. There are reportedly fascist elements in Ukrainian society to which Russians don’t tolerate because of Hitler’s atrocities against Soviets in the WWII. There is also anti-Russian sentiments in Kiev and Western Ukraine.
- There are many other subtle reasons why Russia can’t leave Ukraine alone.