Russia, Belarus, Ukraine. Those three countries are the cultural and historic heartland of the russian people as a whole.
Ukraine broke apart and to save this "cultural" Russia, russian paramilitaries invaded Crimea and the russian military is supporting the rebels in Eastern Ukraine.
"It's okay. It's okay. Calm down. We have saved Crimea and some of Ukraine. Cultural Russia is still somewhat intact. Battered, but still valid."
And don't worry: Russia still has Belarus! Russia is still important! Russia is still whole! Russia is still an empire!
Now imagine the freakout in the country Russia if Belarus, the only other remaining member of cultural Russia, decides to take a step backwards, away from Russia.
Without Belarus and Ukraine, what happens to cultural Russia? (Imagine the New England-states of the US seceding.)
And just as with Ukraine, Russia would think that this break-up was engineered by people who want to hurt Russia.
"Ukraine would never leave us! Somebody made them leave us!"
"Belarus would never leave us! Somebody made them leave us!"