There's obligation also, responsibility.
The deal was n 1991 that Ukraine was hands off for NATO, and we (the West) would protect them if they gave up their nukes.
We screwed up, offering that future membership in NATO. The result was Russia's invasion.
The response is our promise that we have to keep. And not just because we said we will protect them, but because we are the cause of the invasion.
So it isn't just a matter of simply stretching ourselves thin for the sake of an ally, as the opponents of assistance would put it.