it's an extremely complicated issue, metaphysical basically. and there are plenty of reasons. historical, cultural, religious. economic and security reasons are actually secondary. russian empire spent centuries to gather all ancient rus' lands. and didn't even manage fully, it was ironically ussr which did, but not for long. same shit again. russia can't have kiev in western sphere, this makes the country incomplete. that's why it's laughable when people bring up finland and its accession to nato. it's incomparable.
Ukraine