There's a lot of propaganda out there from both Western and Russian media outlets. Time to cut through the crap and report on the facts!
NATO has been expanding eastwards since the collapse of the USSR. Rather then draw up a new European security arrangement to include Russia and further demilitarise the continent, NATO expands to include former Soviet republics and Soviet satellites. Where Moscow was once thousands of km from NATO, it is now only hundreds.
US withdrawal from ABM Treaty without offering Russia legal guarantees it won't be used against them. It may be quite limited now, but a few decades from now it may have the capability to knock out Russian ballistic missiles so in effect giving the US a first strike capability against Russia without fear of retaliation.
Unilateral military campaigns by the West bypassing the UN and thus making countries that do not conform to US political and economic ideologies (i.e Russia) nervous.
Russian imperialism against Ukraine. Ukraine has suffered tremendously under Russian domination and it's no wonder the majority of the population want to look West and join NATO to protect themselves from the corrupt authoritarian Russian bear.
Putin is part of a long list of Russophile authoritarian leaders who have a nationalist and paranoid outlook on geopolitics and to combat perceived threats want to expand Russian influence especially if they see Russian national security under threat at some point in the future. They wish to recreate parts of the old Soviet Union under the Customs Union and CSTO banner and Ukraine is absolutely essential, it's practically Russia's under belly. Take that out and Russia would be extremely exposed to armoured formations making a quick dash for Moscow due to the flat Russian steppes. Western tanks dashed through the flat Iraqi desert. Now imagine how much easier that would be on grass (See Operation Barbarossa). The further NATO is away from it's borders, the safer the Russian elite feels so it will push outwards like it did under Imperial Tsarist Russia and like it did under the USSR banner. The West is trying to prevent Russia from dominating it's neighbours again and bring countries such as Ukraine under the EU style political and economic development (starting with a hefty dose of capitalist shock therapy- which in the end didn't work on Russia which is a more Eurasian country than a European one).
