The basic answer is that Russia doesnt like that Eastern European nations like Poland and Ukraine are part of Nato or US influence. Also just as important, although Putin knows he cant rebuild the Soviet Union in terms of land size, he still wants these countries to be under the influence of Russia. He doesn't have to run the countries which is more of a headache than it's worth, but he wants them to be part of the Russian alliance as its border against Nato. Honestly, i don't understand why he would ever fear Nato. Nato was basically created to defend against the Soviet Union. I can't honestly believe he thinks that all of the members of Nato would one-day vote to invade Russia.
In any case, there is zero percent chance that the US or Nato would defend Ukraine against the Russians. At best, there would be economical penalties and i am not even sure that would even amount to much as countries like Germany buy a lot of energy from Russia. The world allowed Russia to take Crimea without much of a penalty, so why should Putin worry about Ukraine. He will probably take out the leadership, put in his own people and Ukraine would be once again an independent nation with a leader who supports Russia
Russia views any country or alliance that is not part of or controlled by Russia as an enemy. The Russians are paranoid and insecure.
Of course they have good reason to be so. Their history is replete with foreign invasions. The Swedes, the French, the Turks, the Poles, the Japanese, the Germans (ESPECIALLY the Germans), etc. have all invaded Mother Russia in just the last 400 years.
Ukraine was much a part of Russia as Scotland has been a part of the UK, up until 1991.
Putin (and much of Russia) misses its Empire and wants it back.
Whether or not Ukraine being reabsorbed into the Motherland is a threat to US national security is a problematic question. Same as Taiwan being reaborbed into China.
Americans were once told we had to fight the Viet Cong in the Mekong Delta so we didn't have to fight them in San Diego.
Americans were once told told we have to fight the Iraqis in Iraq so we didn't have to fight them in Brooklyn.
Lies, on both occasions. At tremendous price.
I don't want to see freedom-loving Ukrainians fall under the Russian boot, of course. And I feel bad for the Uighurs & Tibetans in China.
But enough, already. If Europeans don't care about Russian expansionism on their doorstep, why should Americans?
Narco-terrorism in Latin America is a much more direct threat to US national security. And has been for 20 or more years. If we had spent more attention to events in Mexico, Honduras and El Salvador as we did Iraq, Syria, Iran, Yemen, etc. would we be better off today?