Putin is going to war in the Ukraine because he fears because he and Russia fear being surrounded by NATO forces. NATO was specifically organized to counter the Soviet threat to Europe. And in the past few years, NATO has welcomed many past ex-Soviet allies to join NATO:
Enlargement of NATO - Wikipedia And this scares Russia to death, considering WWII and the threat that Europe/Germany posed to Russia.
Putin and the Russians are scared and seek to procure security for their country. So, this is the reason they are seeking to secure the Ukraine as a buffer zone and protection from Europe.
Just my opinion.
Putin has said why. I don't see a reason to doubt his statements. Most of all, he resents the loss of the former Russian empire. He is an entitled, narcissistic and arrogant bully. He has around a million soldiers, 4 times the size of Ukraine. Hitler told the world exactly what he planned and the world pretended that he didn't really mean it. How wrong can you be?
The question I ask, is if Putin occupies Ukraine, where will he stop? Will he take Poland? Estonia, Latvia and former Soviet satellite countries? He does not need the territory. Russia is huge as it is. His pride drives him. It's straight out of the dictator's playbook. Divide your enemies - Germany is a weak link. If Russia gets its way, will China decide to invade Taiwan? Probably. Why would they not? That would push the US out of South East Asia. Putin knows full well that NATO is not going to start anything. If NATO had wanted to, they could have crushed Russia long ago, before Russia's economy had recovered from the cold war.
For those who are the victims of war, it's a tragedy. For the non combatant nations, they need to consider the consequences of standing on the sidelines. If Ukraine falls, how about Taiwan? And why would North Korea hold back from invading South Korea? If America is OK with that, who can complain? It does mean that America retreats from the world stage. We will have a new new world order. Not something that look forward to.