1. Russia has a deep seated psychological worry about invasion from the west across the open plains of Eastern Europe (Poland/Ukraine) (Napoleanic wars, Ww 1, ww2) so creating a buffer is of geostrategic importance (old Soviet Warsaw Pact / satellite states. Merely having a nato member bordering Russia may stoke this anxiety (if Ukraine were to join nato)
2. The window for action is closing driven by a)population decline b) decreasing foreign reliance and wealth created from Russian energy wealth (oil/ nat gas)