Actually this is very clear to me as I was there a few times and talked to my friends on both "sides" - that is, from both Eastern and Western Ukraine. They had very different views.
The people from Donbas were very poor. Actually Ukrainiansin general were poor but e.g. in Donyetsk there were whole districts where people stopped paying for electricity etc. They had the idea it would be better to live under the Russian rule. Same in Crimea: there were mostly Russians or people speaking Russian everywhere. They thought that if Crimea was Russian they would be better off as more rich Russians would come there.
Western Ukraine was very different. I asked someone on the street in Russian and he kindly answered this is not Russia, they have another language etc. I was surprised as I didn't mean to start a debate, I just wanted to know my way to the train station.
Back to your question. It's very clear why the war started and Putin explained it in very simple terms in the victory piece that was published by RIA and quickly retracted[0]. In short, he felt like Peter the Great and wanted to take over the whole Ukraine to be remembered in history books as someone who gathered both Belorussians and Ukrainians under single Moscovian rule.
[0] https://www.bbc.com/news/technology-60562240
