The article gives you a brief idea:

Kievan Rus was where, over 1,000 years ago, the Russian state and the Russian Orthodox faith began. That is why he will do his utmost not to let it go, whatever the cost.

More than that, the areas of Ukraine became very tied to Russia during the Soviet Union. It was, after all, part of the Soviet Union for more than 40 years. Most of the area in eastern Ukraine, and Crimea especially, is composed of ethnic Russians and supporters of Russia. There's also the whole Holodomor, where the Soviets basically encouraged a famine (intentionally or not), and the consensus on that (as far as I've seen) is that the Russians singled out the Ukrainians. They've had a long relationship, Russians and Ukrainians, and are very ethnically and culturally tied, and as early as 1991 were still united under one banner.