Crimea was annexed after entities openly hostile to Russian interests claimed political power in Ukraine.
Notwithstanding whether or not the people of Crimea were okay with said annexation (because some sources say yes, others no, and the remainder are more ambiguous): given that Crimea holds Russia's only year-wound warm-water port (historically speaking), it is no surprise that Russia would seek to guarantee access to Sevastopol in the brief power vacuum that resulted after the Maidan Revolution.
Any other country in an analogous situation (America included) with an interest in power projection and being a Great Power would have done the same.
What a complete rationalization. Friendly and hostile governments come and go in all countries. Ukraine is not a vassal state of Russia in which Russia has the right to a friendly government. Nor was Russia’s naval base at Sevastopol threatened by the new government. Rather, Putin coveted Crimea and used the events in 2014 as an excuse to grab it. Overall, as he has indicated in multiple addresses, Putin does not accept Ukrainian independence and wishes to correct Lenin’s big mistake and reabsorb it back into Russia, regardless of the Ukrainians’ desire for independence. Everything else is just an excuse for this ambition.