Ukraine has deep cultural, economic and political bonds with Russia. Many Russian speakers there. It was the second most powerful and populous state in the USSR (the first being Russia). It held much of the USSR's agricultural production, defense industries, and military, including the Black Sea Fleet. Ukraine's decision to sever ties with the USSR in 1991 started the dominoes falling for the end of the USSR.
But, Ukraine has been moving more and more towards the west, and wants to be a part of the EU, and of NATO. A westernized Ukraine leaving the Russian sphere and becoming a part of the EU is considered a destabilizing threat to Russia. And, reuniting Ukraine with Russia would bring Putin one step closer to restoring the greatness of the Russia in a form similar to the Soviet Union. (He already has Belarus pretty much under his control.)
Will he do it? Of course. He has already done it once. Russia now holds two huge sections of Ukraine, and the West did nothing, in violation of the promises the West gave Ukraine when Ukraine agreed to give up its nuclear weapons to Russia. (As an aside: You know that North Korea and Iran were watching that...and how Obama abandoned Qaddafi after Qaddafi surrendered his nuke program to George Bush.)
Will he go further? Who knows? If China moves on Taiwan, Russia may believe it can move on Ukraine safely. Again, who knows?