Montenegro was one of the last few bastions of pro-Russian sentiments in Eastern Europe. The Balkans with its Slavic Orthodox heritage can be seen as the natural cultural brothers and allies of Russia. Toynbee regards the Russian civilization as a descendant of an older Orthodox civilization, and no doubt the two civilizations have many traits in common. Whereas the northern parts of East Europe are integral parts of the Western civilization, this is not necessarily the case for the southern parts, which are culturally closer to Russia.
Russia has been pushed very much eastwards and reduced in sphere of influence. The west is expanding ever more both politically and culturally into the east.
Russia could have hoped for support from countries in the orthodox parts of ex-Yugoslavia. Bulgaria and Romania have already been lost partly because of the anti-Soviet heritage. The post-Yugoslavian states are following suit in the hope of richness. Montenegro was one of Russia's last allies. Its switch of allegiance to the West will be felt as a humiliation in Moscow.
Of course Montenegro has the right to self-determination, but it is nothing less than arrogant that the EU and NATO keeps on expanding at the cost of Russia. It is also short-sighted: The cold war is cooling further, and the fight against terrorism is being disturbed.