Grabbing the port and demonstrating to Russians in Ukraine that Russia would not surrender their region to NATO was set off by NATO expansion. Imagine Russia moving missile defense systems to Mexico or Cuba for a parallel. It is strange how Americans construct these narratives. People still believe the September 11 attacks were a sucker punch out of nowhere instead of Al Quaeda attacking buildings that they tried to blow up before long after they declared war on the US. With Putin the west behaves as if they were just minding their own business in the world and this big bad man just attacked another innocent country for no reason whatsoever.
You don't have to hero worship Putin to acknowledge that he is coming across as a more skilled statesman with a far more coherent approach to the ME.
What NATO enlargment are you talking about? NATO hasn't expanded since 2009 nor was there ever any real discussion about Ukraine joining NATO prior to the Russian invasion of Crimea. At most, a few member countries talked in very vague terms about how they might potentially welcome Ukraine into the fold at some point in the future but those sorts of statements have been common since the end of the Cold War. There was no reason for Russia to worry about losing access to Sevastopol either, thanks to an agreement they had with Ukraine that guaranteed Russian access to the port until 2042.
"NATO enlargement" doesn't explain Russia's action in Ukraine. Instead, I think it's more about Putin trying to re-establish the Russian sphere of influence. Which also helps explain the burgeoning coalition in Syria.