If the Ukrainians are unwilling to do it, why should we?
Today, nobody will. Tomorrow (Western Ukraine), nobody will. The day after that (pick a non-Nato Eastern European country) nobody will.
Next week, when Puti goes for The Baltics, will someone finally have the nerve? Putin's calculus will tell him "No". And he may be right.
Next Month, Poland?
If not today, at which point will anyone stand up to the bully? Never?