This has been going on since before Biden was in office. Although, Biden is 100% guilty of escalating this at this moment.
1. Ukraine owes Russia a TON of money. Hundreds of billions. They've been free loading off of Russia since Ukraine was created. They're reneging on their promise to pay.
2. They're also sucking up tons of money and weaponry from Russia's enemies. (Like the US, Obama, Trump and Biden)
3. NATO is surrounding them.
4. Ukraine's government is corrupted by far right wing nationalist.
5. Instead of repaying their debt to Russia, they run to their big brothers in NATO and plead innocent.
And since the US always has to have a boogey man, (that always being Russia). Our leadership has never really wanted Russia as an ally. The threat of Russia helps to keep our military industrial complex in business. Notice the lack of aggression in the middle east lately? I didn't notice either, because of the war drums beating for Russia.
With NATO on Russia's door. Ukraine screwing Russia over. This has left Russia on the defense.