Russia would be brain dead if it didn't sharpen their defensive readiness when faced with the extreme aggression of the western nations.
NATO's military budget is 20 times bigger than Russia's, if we exclude the US, NATO still has 5 times bigger military budget than Russia.
Our rhetoric is quite insistent on continued military build up in Europe.
Russia has 6-10 military bases abroad, the US and its allies, mostly the US though, has around 1000....
Western aggression, where should I start...
Experimenting with two nuclear weapons on Japan even though it was clear the war was won at the time, the korea War, Vietnam War, Gulf wars, bombing of Serbia based on a lie, agreeing to Kosovo being a autonomous part of Serbia in the peace-deal and then breaking that agreement afterwards, invasion of Afghanistan without a rational reason, invasion of Iraq based on a lie, bombing of Libya based on a lie (this destabilized the entire African continent by giving Jihadists a safe haven there too), a committed support to the extremists who took to arms in Syria, Turkish occupation of northern Syria, American presence there, American betrayal of the Kurds when Turkey wanted to murder some Kurds, deadly American bombing in 2019 that killed about 70 Syrian women and children.... Not to mention all the atrocities the US does in latin America, a whole chapter in itself; supporting coups, committing assassinations, funding warlords and dictators, and so on. Western aggression towards Iran is another topic of note, claiming Iran to be supporters of global terrorism, name one shia group of note that belongs to the global terrorist category, it should be mentioned that Shia's are one of the main victims of Sunni jihad.
Assad, Saddam and Ghadaffi were all inside the western sphere of influence, extended friends of the west one could say, up until our aggression. They all lead secular regimes although at least two of them were personally very religious, but their common mistake was to meddle with something described as Islamic socialisms, nationalizing too many industries for the west to swallow. The US created Taliban (also Al Qaeda) when opposing the secular regime that the soviet supported in Afghanistan, that regime's mistake was also socialism one could say. Let's not forget how Israel yearly sends missiles into its neighboring countries, their war against Lebanon mainly because Lebanon housed hundreds of thousands of Palestine refugees (equates to them supporting terrorism I guess).
Almost forgot to mention the western support of the Saudi Arabian mercenary war on Yemeni farmers, mostly US support though.
Georgia attacked Russia gambling on it being the right move to get NATO membership pronto, the gamble failed because NATO somehow didn't want a war with Russia just to stop two Georgian regions from gaining the autonomy they wanted. Ukraine is suing for NATO membership in blatant disregard of the buffer state concept that's been in place since the dissolution of the soviet union. Azerbaijan, another buffer state, recently waged war against Armenia with Turkish support, is another example of western aggression that worries Russia.
The US knows only one kind of diplomacy, and it is based on threats and uncompromising ultimatums, and their only response to failed diplomacy is aggression, if you're not our b*tch, then you're an "evildoer". A recurring precondition demand for negotiations that the west presents is "lay down your arms and accept defeat", and when the demand is not met we accuse the other side of not being willing to even negotiate. What kind of negotiation is possible or even needed if the other side has capitulated before the negotiations, what power does the other side hold in that negotiation? This occurred during the east Ukrainian civil disobedience and again in the following civil war, it occurred in the Syrian civil war and in the Libyan civil war, it is also similar to the demand made against Cuba, Venezuela and so on as a condition for better relationship with the US, accept all our demands "kind-of-thing" and we might allow you into the room.
So considering how the ultra aggressive, imperialistic, US, has been increasing its military presence in Europe on the Russian border lately, new missile sites, the anti missile shield, boots on the ground, along with NATO's general arms build-up, it is only reasonable that Russia has a strong and even paranoid military presence on their borders to NATO, or whatever proxy that may be (and Ukraine is starting to look de facto like NATO), i.e. unstable buffer states with strong leanings to the west.
Russia does not want borders against NATO, they desperately want Ukraine to be a stable and predictable buffer state, an invasion with the intent of occupation is unthinkable. However, it could be thinkable that if Kiev breaks the cease fire and commit a full on invasion of the separatist controlled east, Russia might respond militarily (similarly to how they did in Georgia) and be able to justify it enough ("To stop Kiev from committing a massacre against its own people" could be a valid rhetoric) for NATO to not get involved in it (because its a clusterf*ck that would have been initiated by Kiev).
Some of what I said here is of course debatable, especially the semantics of it, it lack some nuance, but this is partly the dominant perspective of the non-western world population today, so refusing to consider this narrative as something real would be a dangerous mistake of ignorance.