I have an answer to a linked question which answers how Russia ended up opposing West, but not why. The answer to latter is actually not known, but I can guess:
We are mostly discussing the standoff between Russia and the USA. Europe (or Japan and South Korea) just did not have that much of a problem with Russia before February 24th.
First of all, USA is famous for not being able to befriend some countries. Examples include Cuba, North Korea and now Russia. One can possibly explain out North Korea from that list, but the fact that USA can't normalize relations with Cuba points to either some fatal flaw of American diplomacy or some hidden force preventing this from happening and exposing USA to the only real attack surface in its vicinity.
Second, Russia looks to be the most convenient opponent to counter USA. If you would be picking an opponent for the USA, but one who would not end the world as we know it, who would you choose? Iran and other Muslim countries have motivation but don't really have any means to scare USA. India is too far to do any damage to USA and does not have motivation. Brazil could find motivation and economically large but military-wise nothing to write home about. China, on the other hand, is too large economically and its military still lacks some capabilities, though it is obviously catching up. Anything in Europe is too interdependent on the USA economically and too vulnerable for direct American military influence. USA - China or USA - Europe standoff would cause world economy to crash hard.
Russia is in the perfect spot, so it seems - serious military with all kinds of stuff available, including some fleet, an air force and long-range rockets. Russia's economy may survive the conflict for some period of time but sufficiently firewalled from the global economy as to not crash it immediately. If you wanted to put some check on the USA it would be Russia.
Thirdly, Russia (and ex-USSR) is a part of "ring of fire" around Europe, comprising also of Maghreb countries and Middle East, where all kinds of wars tend to happen in the last 30 years. This supplies the necessary military action, which is not there even if you consider China - USA standoff. How the ring of fire came to be is another question.
This still does not answer the "why" question - the potential is there but motivation for both conflicting parties is hard to reason. In that sense it looks more a bar brawl than a conflict of interests - USA and Russia just don't really have much conflict of interest.