Putin cannot declare victory in Ukraine... because he shifted the goalposts for victory too far out.
Putin still thinks like a secret-agent. The "Special Military Operation" was supposed to assassinate Ukraine and be done with it before anyone could do something about it. Two problems:
1. Russia's FSB fed intel to Putin that Russia would be greeted as liberators because that was what their superiors wanted to hear.
2. Ukraine had 8 years to prepare for a russian invasion.
The first reason to invade Ukraine was given in 2019:
"NATO is encroaching militarily on Russia via Ukraine and Russia needs to preemptively invade Ukraine to prevent that."
About a day into the invasion, another reason was given:
"The US have been developing bioweapons in Ukraine, for use against Russia."
Actually, the US entered a contract with Ukraine in 2005: To develop countermeasures against soviet-era bioweapons and to prevent them from falling into the wrong hands, e.g. Al-Qaeda.
When the international community did not buy that, another reason was given:
"Ukraine is secretly ruled by Nazis and we invaded to denazify Ukraine."
Ok...
So far, the russian government has named 3 conditions for declaring victory:
1. stopping NATO's expansion
2. demilitarization of Ukraine
3. denazifying Ukraine -> toppling the ukrainian government and replacing it with a puppet
That would be hard enough in and of itself, but various russian politicians, russian generals and russian media-personalities have added ADDITIONAL goalposts on top of that.
* A member of the Moscow city-council has floated the idea that Russia should denazify the Baltic states, Poland, Moldova and Kazakhstan next. (Which is weird because Kazakhstan is solidly pro-Russian except for some anti-government protests.)
* A journalist has suggested that Russia should attack and conquer a swedish island in the baltic sea and shut down european airspace from there.
* One russian general has said that Moldova is next.
* Another one has said that Poland is next.
* Another one has said that Russia needs to demilitarize NATO.
* And the russian population is saturated with propaganda that this is WWIII and that the West is trying to destroy Russia.
As far as the russian people are concerned, victory has been achieved once Russia has:
4. conquered all of Ukraine, not just Donbas, Luhansk and Crimea
5. conquered the Baltic states
6. conquered Moldova
7. conquered Poland
8. conquered any other ex-soviet country that is too independent:
8a. Kazakhstan: Has had anti-government protests against the pro-Russian government. And the pro-russian government recently refused to hold a joint military parade with the russian military, hinting that they don't like this russian attitude that ex-soviet-countries have no right to exist.
8b. Georgia: Georgian mercenaries are fighting alongside the Ukrainians. And Russia has already cut a region out of Georgia and installed a puppet-government there.
8c. Kyrgyzstan: It has hinted that it wants to leave Russia's "Eurasian Economic Union". (Most likely in order to escape the economic sanctions. Also, reports have surfaced how badly the russian government treats kyrgyzstani immigrants, including forcing them to join the russian army under false pretenses.)
9. driven NATO out of Europe / demilitarized NATO
Basically, Putin cannot simply declare victory and stop once he has conquered Donbas and Luhansk. Because russian propaganda has fed totally unrealistic expectations to the russian people.
Putin is doomed to continue fighting, because any other resolution than the destruction of NATO and the conquest of Eastern Europe would make look him like a weakling, a liar, a loser in the eyes of the russian people.
And Putin would rather send tens of thousands of Russians to their deaths than look like a weakling, a liar and a loser.
= new reply since forum marked as read
...Putin attacked is because the EU declared it will be carbon neutral in 28 years~by 2050, killing the Russian's cash cow.
In recent years enormous amounts of gas and oil have been discovered in the eastern region of Ukraine, which Russia has now 'claimed'.
Unlike the other oil rich, land locked countries in the region, Ukraine has water access to export theirs. Gas pipes to the EU is a fraction of the distance Russia now maintains.
This impending competition with the Russian supplies during this closing window of the oil era was deemed unacceptable.
Conveniently, these supplies makes nice collateral for the US lend lease program with Ukraine.
Having said that- fuck Putin and his thieving ass lickers. I dream of a world where Ukraine wins and becomes such a wealthy, happy country that the Russian people choose to dissolve their country and become Ukrainian. 
