Russia wants to steal Ukraine for military and economic strategic reasons including natural resources, land routes, additional sea ports and human resources. To the Russians, Ukraine would be another gateway to conquer Europe as a next step.
Besides other important natural resources, Ukraine has good land for very large grain crops. Russia desires to control and eventually own all of Europe, but that desire would not stop with Europe.
1973 United States–Soviet Union wheat deal
https://en.wikipedia.org/wiki/1973_United_States%E2%80%93Soviet_Union_wheat_deal
Governments like the government of Russia tend to control their populations by vilifying and invading other nations, and they don’t willingly stop expanding. Only other nations can hold such compulsive bully nations at bay, and eventually running out of room to compromise, stop them.
I’m not convinced “Russia” wants to steal Ukraine.
The average Russian person doesn’t want Ukraine - they think they should be “reunited” but not forcibly.
The Russian governmental establishment wants Ukraine for historical reasons.
Russia - or to be more specific Muscowy - has a historical “concept” of itself as
1. The uniter of all the East Slavs
2. Inheritors of the Constantinople empire
With #1 they expanded it to pan-Slavicism in the 1800s, which failed.
With #2 they expanded it to pan-Orthodoxy in the 1800s, which also failed.