There is a couple reasons. In 2008, Ukraine discovered multiple areas rich in oil near the east and west borders, and in the oceans around Crimea (Map from RealLifeLore), making Ukraine a potential competitor in the European oil industry, which is where most of Russia's profits come from. If Ukraine becomes a competitor, it would hurt the Russian economy. In response to this discovery, Russia has annexed the island of Crimea to add it to its own territory. The other reason is because of a possible threat from NATO. In the 2nd mad, you can see the Russian Territorial borders in orange, and the NATO borders in blue. If Ukraine (Yellow) joins NATO, Belarus would be near impossible to defend against being surrounded by NATO territory on 3 sides, but if Russia had Ukraine, it would be significantly easier to defend Belarus.
