In short, Russia’s economy is stagnant. Rather than fix the problems and address the corruption from which he personally benefits, Putin has discovered that it is easier to whip the flames of nationalist fervor. But every time he makes a land grab–Abkhazia and South Ossetia in 2008 and Crimea and perhaps soon eastern Ukraine in 2014–he must subsidize the new territory, creating an even greater drain on Russian resources, all the more so since he also subsidizes client states like Belarus to keep them in line.
Therefore, with every territory he grabs, the speed with which the Russian economy unravels increases, forcing the need for even more land grabs to stay ahead of the issue. It’s analogous to a cocaine addict who must constantly up his dose to get the same high.