Putin probably remembers his history. In World War II, Germany nearly overran Russia, and it started from the middle of Poland. If it had been able to start from the eastern border of Ukraine, Russia might well have not survived.
So Putin may feel, historically, that Russia's survival is at stake if Ukraine joins NATO. (I am not saying that Putin's desire to control Ukraine is moral or ethical. I am merely saying that it is understandable.)
Putin probably remembers his history. In World War II, Germany nearly overran Russia, and it started from the middle of Poland. If it had been able to start from the eastern border of Ukraine, Russia might well have not survived.
So Putin may feel, historically, that Russia's survival is at stake if Ukraine joins NATO. (I am not saying that Putin's desire to control Ukraine is moral or ethical. I am merely saying that it is understandable.)