So sad in todays day and age that a Putin would do this?? But who's to say who all are behind this, even some Americans getting wealthy from it??
Edit: Oops. Olive beat me to the vid.
Keep in mind, the video is an analysis and not an defacto answer to the question as to why. To sum it up if you don't watch the vid (but i recommend you do) Eastern Europe has a wide plain that acts as a funnel towards the west. When the USSR was in it's prime, it could easily defend against western troops by creating various choke points. When the USSR broke up, those point disappeared as countries became more western aligned and Russia was pushed east. That made the area that Russia would need to defend against in terms of a land invasion MUCH wider. By getting Ukraine and Belarus, they get some semblance of the choke point back. Russia is VERY concerned about a land attack coming from the west toward Moscow and defending against it.
Also, Ukraine was found to have large deposits of natural gas, in the east, and off the shore of Crimea. Coincidentally if Russia annexes the eastern portion and Crimea, they control most of the deposits and get to profit from it. Ukraine also charges Russia a

load in tariffs for pumping gas through the pipeline in the country. They were working on freeing themselves of this burden, but that pipeline was to finally close sometime end of 2024. For now, they pay a lot of $$$ to Ukraine.
Even shorter version. Taking over Ukraine gives Russia valueable resources, and a better defensive posture against a Western land invasion
My take on this is that the best Ukraine can hope for is to give up all hope of Nato membership and pledge to remain neutral with various promises to never host foreign troops. I doubt they will ever see the annexed lands back due to the $$ in those resources under the ground/sea.
As to why Russia is so paranoid. No idea. You'd think the idea of nuclear war if NATO ever crossed the border would be enough incentive to never try it. Makes me wonder if Russia's military is so underfunded/corrupt/inflated, that their nuclear deterrent isn't as great as we think it is. These things have a shelf life, and must be recycled and renewed, and I don't see a country with the GDP the size of Florida maintaining an arsenal that would rival the US. I wouldn't be surprised if the truth is deception as to the capability/size. However i'm not willing to call the bluff. Just stupid keyboard warrior stuff here so take this all with a grain of salt as I am not qualified to speak on the subject.