I'm no expert, but apparently Putin is somewhat obsessed with restoring the Russian Empire of the Czars.
After the Soviet Union fell apart, a lot of little nations went their own way. Ukraine is one of these. Although Ukraine does have some history as an independent nation, Putin looks back at a fairly substantial period of history and concludes that Ukraine simply belong as part of Greater Russia. It is his mission to restore Russia.
I'm not sure Putin really wants to be crowned as Czar, but he does seem to have that sort of goal. I hear that Putin has cancer, so maybe he is just rolling the dice -- if he expands Russia, he's a hero at home; if it goes horribly wrong, he'll be dead anyway, so it's not his problem. I think he's "going for it".
Thank you.