They need legitimacy on the world stage, and see themselves as the natural succesor to the power vacuum created by the fall of the Soviet Union. Basically they achieve nothing by allying with the US and can be seen in a more respectful light in the world by opposing us. It's nothing personal, and behind the scenes the US and Russia have grown closer since the end of the cold war. You'll notice Russia has said hardly a peep about the war in Iraq/war on terror. Lots of stuff is going on behind the scenes.. trust me.