Jens Stoltenberg just admitted what many of us have been saying since the start of Russia's invasion: This was caused by NATO's unending expansion towards Russia.

That doesn't make what Russia did "right" but it does explain why. No major power (I don't know if I'd call Russia a "great" power at this point, but "major" for sure) will allow what they see as an existential military threat to develop on their border. We (the US) don't allow it, we shouldn't expect that Russia would allow it either.