We read that Russia invaded Ukraine because it doesn't want a democracy or a NATO member on its border. But I'm more inclined to believe this is the big reason Russia wants control of the Eastern and Southern regions of Ukraine, and Putin is not going to give up those areas easily.
"From Price Wars by Rupert Russell"
"In 2012, enormous gas reserves---2.3 trillion cubic meters---were discovered under Ukraine’s share of the Black Sea. Russia tried to negotiate access to the deposits, but the talks fell through. Then, in January 2013, Ukraine struck a deal with Royal Dutch Shell to start drilling in Eastern Ukraine, where another major deposit of natural gas had been discovered. In April 2013, the Ukraine energy and coal minister declared that the projects in Eastern Ukraine and the Black Sea around Crimea would soon start producing so much natural gas that the country would no longer need to import it from Russia or anywhere else. In fact, Ukraine would become a net exporter to Europe---competing with Russia---by 2020.
If these plans came to fruition, Putin’s leverage over Europe would evaporate. He had no intention of overseeing the same unraveling of energy dominance that the USSR had suffered in the 1980’s. But Putin still had time. Ukraine’s plans for energy independence had yet to become a reality."