I read an article recently—which I'm afraid I can't find anymore—that posited that part of what's going on is that Putin, and some of the rest of Russia, has a rather surprisingly expansive view of what it means to be "Russian", combined with the belief that if one is, by that definition, Russian, it is good and right to be part of Mother Russia.

It also pointed out that by extension, this means that anyone in Ukraine who does not believe that they are Russian (and thus does not want to be part of Russia) is either effectively insane, or some kind of seditious traitor (I don't remember the exact way that logic went in the article at this point).

I don't know to what extent what it described is true (my guess is "somewhat, but not necessarily absolutely"), but it does seem to help explain some of what's going on.