The extreme stakes make it far more likely that he wants the expansion he made the declaration of war speech about (and even more to crushing the idea of a potential successful large democracy full of Russian speakers), not an empty promise concerning something that wasn't likely to happen anyway and is of little interest except in ensuring ease of invading Ukraine. The Ukraine border is of zero strategic value to NATO since they're not remotely interested in or capable of mounting a ground invasion against a nuclear power whose terrain is where world conquerors go to die (and pretty much the only conceivable circumstance where they might have done it is in response to Russia invading Ukraine... the thing he actually did). They don't exactly have happy memories of fighting in Finland either, which is where their border with a newly galvanised NATO will end up instead.
It's even more transparently bullshit version of the preemptive defence doctrine than WMDs in Iraq, to the extent he had to manufacture a secondary excuse for domestic consumption because even a public that buys the idea he's shelling Kiev to defending the Donbas from atrocities won't believe he had to do it to save the country from NATO invasion.
* The sheer overwhelming strategic significance of the Ukrainian border. It is where Russia almost lost to the Nazis.
* His willingness for Russia to endure such an extreme cost. Russia will pay dearly for this invasion and we have watched him for 20 years enough to know that minimizing cost and acting in a rational, measured and brutal manner is the norm.
* The fact that if this goes badly he could very well have his head on a stake. Russia paying dearly very much puts his personal safety at stake.
I wouldnt believe a deadbeat liar telling me that he'll have my money by Friday, but if he's telling me to back off or he'll punch me I will.