I am certainly no expert, but the simplified reason I heard was that Nato had considered offering the Ukraine membership some time back and Russia considers that a threat.
When I heard that I thought, seeing what Russia is doing now certainly proves they probably needed to be a part.
Yes, it is true. Nato once said that whant nothing to do with eastern states but actually did the opposite and actually got really close to Russia. As may know Russia and Nato are nit great fans specially Usa being a big part in the NATO decisions