Russian history is essentially the story of Russia getting invaded time and time again. The Mongols, Napoleon, Hitler, the list goes on and on and on. This, combined with borders that are difficult to defend (no mountain ranges or other natural barriers), has left the Russian psyche with a sense that their position is always precarious. It seems reasonable to me, given Russian history, and the fact that less than 100 years ago their society was nearly eradicated by Nazis. So the Russians are always wary of other powers and are always concerned about their borders.
Now, since the end of the Cold War, Russia has lost quite a lot of territory. This territory has all been snapped up by NATO, even though the Cold War was apparently over. NATO has now slowly expanded to the point that it is right up on Russia's border in the Baltics. The Russians look at this, and look at the failures of "shock therapy" economics in the 90s, and look at their demographic crunch, and look at the increasingly unstable political situation of their Western neighbors. They conclude that now is the last and best possible time to make an aggressive chess move against the West, and snatch up Ukraine before the West can.
So that's what this is about - it's a response of geopolitical insecurity. Russia thinks the West is very aggressive and unstable, and seeks to put Ukraine in its "sphere of influence" before it's too late. To be honest, I don't think Russia is in the wrong here, there is no real reason why Americans should even be involved with this country right up on the Russian border. How would Americans feel if we lost the cold war and Russia started trying to dominate Mexico and Canada?