Shit guys, it's all about money. Put simply Russia has been robbing the former soviet territories into the ground. If the state of Ukraine were to join the EU, as roughly half of the populace does, Russia stands to lose not only a significant amount of capital but also several major naval ports. Actually, more than money, it is likely all about naval power. Which would tie in with their attitude about the recent conflicts in Syria and Georgia. The next big reason is that Russia does not want European powers to get right up side it's borders. Russia has essentially used Ukraine and all of it's border-states as meat shields from western aggression for nearly a century. There is also the fact that Ukraine might still have things, unknown to us, that Russia wants. From weapons to vodka.
Oh and yea, Putin is a Dick.