Russia:
- will not be able to maintain such a large army in the future due to declining population
- can greatly reduce the land border that needs to be defended by expanding territory to better make use of geographical features
- wants more of useful habitable territory, most of it is cold snowy trash
- from the russian perspective, it's now or never
- plus the salty ppl like putin wishing they'd never lost the territory to begin with
If any of these points were appropriate then 2014 invasion would not have stopped at a fraction of Donbas region.