Europeans yearn for the trenches
This brinksmanship is of course extremely dangerous, but there's at least one optimistic explanation for it. Which bargaining position is better for Ukraine:
- We're losing the war on the ground and our allies are abandoning us, or
- We're losing the war on the ground but our allies are seriously considering major escalation?
If you can't walk away from a negotiation because defeat is inevitable, you're going to give up a lot more, and the other side might wonder why they're negotiating at all. But if you have a credible threat of fighting on -- even if your allies don't actually plan on getting involved themselves -- you're far better off in negotiations.