It’s all fundamentally dead simple. Russia views Ukraine as a buffer country between itself and the West and will do anything to keep it that way.
Which was, funnily, in response to US military presence in Turkey, which is still there. It's understandable that Russia doesn't want to see itself surrounded by "enemies".
They should probably stop invading more and more of it over time, then.
They should do it for the sake of security of Europe. Ukraine is not capable of controlling "own" territory.
[0] https://blogs.lse.ac.uk/humanrights/2019/02/05/russias-new-s...
Their actions from 2014 until now show otherwise. Ukraine is not a part of NATO but Putin decided to grab Crimea, Donbas, and keep the military activity for over 5 years in order to destabilize the country.
Current conflict is rooted exactly in Ukraine trying to aligning itself with the West. It would’ve been the EU first, Nato second for them. That would’ve created a massive headache for Russia, including the loss of its only warm port, hence the annexation and the invasion. And now Ukraine is in limbo, because the EU won’t touch it with a wooden pole while it is in an active conflict.