Below is a map of the arable land of the former Soviet Union. As you can see, there are not many places to the north and east where you may want to live–unless you have any particular reasons to sustain climatic hardship. Such as manning military infrastructure, working oil fields, mining gold or generating power. Hence the unrelenting push westward of the Russian Empire, initiated by Peter the Great.
An ancillary reason was the obsession with strategic depth. As everyone has heard, both Napoleon’s and Nazi invasions into Russia failed primarily because of the great distances the invading force had to cover before decisive battles. Getting too far from home, their logistics collapsed, resources were drained, and it all ended as a catastrophe.
Any loss of territory in the west and south affects our food production and security much more than in the east. No wonder Russians, liberal or not, go ballistic at the mere thought of NATO troops stationed along the Ukrainian-Russian border.