Russia is naturally not very open about their real motivation for the invasion, so there's only speculation. Based on Putin's previous comments and actions, his being a former USSR KGB and his disdain for the fall of the Soviet Union, it's possible he just wants to be the first czar of thea reunified Russian empire.
For more substantial rationale, [this RealLifeLore vid postulates that Ukraine discovering an untapped oil deposit threatens Russia's hegemony as east Asia's largest oil exporter (a critical cornerstone of the Russian economy), and controlling Ukraine in its entirety instead of just the Crimean Peninsula opens even more trade routes to export said oil.](https://www.youtube.com/watch?v=If61baWF4GE)