Putin is completely irrational and illogical
The two reasons he put forth are NATO expansion and "Nazis" committing a "genocide" of Russian speakers.
Russia effectively ruined Ukraine's chances of ever joining NATO or the EU by starting a forever war in the Eastern part of the country in 2014, because that would risk NATO/EU having to enter that war if Ukraine were to join. He got his foreign policy goals accomplished in that war. So why invade again?
Only 18 civilians died in the fighting in Ukraine in 2021, and this is a mix of deaths from pro-Russia forces, pro-Ukraine forces, and legitimate accidents. So there is no "Nazi genocide" going on. And even if there was, far right militias are only operating in the Eastern part of the country. Why does he need to conquer every single city to get the Nazis?
So there's no good reason for doing this. Putin gave up his economy and the future of his country. He permanently damaged his support both from the oligarchs who are almost always on his side as well as the common people. Russia has lost pretty much all its soft power. The war is not going well at all and the only way Putin can win is if he turns the whole damn place into rubble like in Syria.