A possessive Russia wanting to retain control over its former soviet bloc. A reluctant bride US playing hot and cold in deciding how far to go annoying Russia. Two young democracies that burnt its finger (read anticipating US support) in the past.

Russia is pretty clear with its national security interest: leave no stones unturned if its perceived as compromising Russia interest.

1/ http://www.nytimes.com/2008/08/18/washington/18diplo.html?pagewanted=1&_r=1
 
2/ Getting Ukraine Wrong: John J. Measheimer. New York Times March 13, 2014
 http://www.nytimes.com/2014/03/14/opinion/getting-ukraine-wrong.html
 
3/ http://www.reuters.com/article/2014/04/18/us-ukraine-putin-diplomacy-special-repor-idUSBREA3H0OQ20140418