Like most things the true reason is far more complicated than what those simplified narratives often say. Unfortunately they make good headlines.

Nobody who can to speak publicly knows for sure whether or not Putin alone chose to start the war and exactly what the reasons where. But if you look more broadly than just 24th February 2022 the reasons go beyond just Putin. Even without Putin, Russia (the country not the people) had reasons (horrible as they are) to escalate tensions with Ukraine prior to 2022, it had reasons to start the war, and even today it has reasons to keep the war going. In the West, and in Russia, Putin is the personification of the choices Russia makes. But Russia itself would have reasons even without him.

Those reasons are many, and I wouldn't do them justice to talk about the various theories. But one in particular that I feel isn't talked about enough is the Sevastopol Naval base. Crimea, and it's naval base was the centre of Soviet and now Russian control over the Black Sea. Prior to 2014 Russia was leasing the base from Ukraine, so it has essentially been under control of Moscow continuously since the Soviet era. The 2014 revolution in Ukraine posed a threat to that continuing. Russia's rather extreme solution to that was to anex Crimea. If you look at from Russia's perspective they were put in quite a tough situation and they tried to make the best they could from the hand they were delt. Most people would probably disagree with the use of "best" in that sentence, but from Russia's perspective it was the right choice.

Since 2014, Crimea has been problematic to Russia for various reasons, in particular due to needing an outside source of fresh water, and needing the very expensive new Kerch bridge to connect it to Russia. One of the apparent benefits of the full scale invasion in 2023 was to supply Crimea with water from the Nova Khakovka dam via a canal, and connect Crimea to Russia via land from the North. The reality of the war so far hasn't actually been a net win for either of those things. And recently Russia has been forced to move a lot of their fleet out of the Sevastopol Naval base due to Ukrainian attacks. But presumably Russia is looking long term and continues to hope for a good outcome eventually.

Looking to the future, both sides think they can outlast the other's will to continue and hope to eventually force the other side to back down. Russia's end game now seems to be some form of negotiation or ceasefire where they are able to retain as much of what they have grabbed onto as possible. Ukraine's endgame that they have stated publicly is to remove Russia entirely from their land. But there is also the hope of being truly free from Russian influence for the first time in centuries. Ukraine hopes to come out of this war with a strong military to deter any further Russian aggression indefinitely. And possibly also NATO membership.

Russia hopes that eventually the political will for Ukraine to continue fighting for that aim, and the West's will to support it will dry up. They also hope that if they keep the conflict going indefinitely at a small scale, or if they hang onto some territory, then Ukraine will never be able to join NATO.

Ukraine knows that if things continue as they have for the last year they will eventually win. The open question is how long is eventually, and will it be too long.

Unfortunately this means enormous loss of life on both sides for the foreseeable future. Likely 1-2 more years minimum and hundreds of thousands more dead.