Here are the reasons that I've found so far:
-Political tension in Ukraine since the country’s independence in 1991
-Violation of agreements that Western leaders made at the end of the Cold War not to expand NATO into Eastern Europe
-The U.S.-backed coup in Ukraine in February 2014.
-The coup in 2014 has put Russia in a tense geopolitical position.
-If Ukraine joins NATO, it means Russia would lose an important naval base at Sevastopol in Crimea. Furthermore, allowing NATO to border Russia is not in Russia’s security interests (similar issue with NATO wanting to integrate Georgia, a country that also borders Russia)
Russia’s reintegration of Crimea in 2014, and the decision by ethnic Russians in Eastern Ukraine to secede from Ukraine as the Luhansk and Donetsk People’s Republics. These were Russian responses to the U.S.-backed coup.
-Tension and fighting in Eastern Ukraine between Ukraine’s military and the separatist People’s Republics
-Failure of Ukraine to follow through on the 2015 Minsk and Normandy negotiations.
Feel free to expand on any of these points. I'm not too familiar with any of them
