No one really sees into Kremlin inner circle decision making but there are several plausible hypotheses:
1. Snack that went wrong
As noted above, previous Putin wars (Chechnya, Georgia, Crimea) were in general short, victorious and boosted Putin’s popularity, and strengthened his regime. At least so far.
2. No democracy in our backyard
A possible prospect of prosperous democratic Western-oriented Ukraine is probably the biggest threat to the long-term stability of Putin’s autocracy.
3. Time is not on our side (my favorite)
Contrary to popular belief and the Russian propaganda narratives, sanctions do hurt. The ability of RF armed forces modernization as well as general Russians prospects for sustainable growth is effectively blocked. Hi-tech imports are prohibited and half of the Russian defense industry has ended up in a hostile country. The best time to do something is now, better than, say, ten years from now. Also, the west is still busy with covid, and oil and gas prices are rising. Nothing to wait for.
4. Place in history
Russian history books in general praise the leaders who succeed in the task of significantly expanding the territory of the Russian state, whatever the cost. Ukraine definitely counts and Mr. Putin’s time is running out.
5. Victim of his own lies
The problem with politicians is that over time they come to believe their own propaganda. So long you say Ukraine is no country and Ukrainians are no nation you lost track of what the truth is.
China, among other reasons. Considering that without Ukraine components Russia is unable to complete a helicopter, a guided bomb, several types of tanks, rockets, etc., strategic competition to China is out of the question.
Also, with Green Deal in the EU, Russia’s main source of export i.e. hydrocarbons is doomed in 10-15 yrs time.
The time is mercilessly against Putin and the window of opportunity was closing. Ironically, with this already lost war, he has achieved exactly the consequences he tried to avoid. Those are the decline of Russia from great power status, acceleration of EU energy revolution, and Russia’s de facto vassal status to China.