Because it's nearly always about natural resources, and no one ever says it. Vlad didn't say so (in terms of reasoning, he never would, but it "just happens" to be what I said). However, natural resources like these are Russias main strength.
Do you REALLY think that he would invade a country with a pretext of NATO when they a) already have NATO as a border, b) will have even more NATO as a border afterwards if they have to protect Ukraine, c) even has the US currently as a border when it comes to it, d) create a new hatred of your homeland from a neighbouring country ? It never made any sense, so you have to think then why.
These people aren't stupid (they may have misjudged the opposition for sure, but their motives are still the same). It's trying to control more resources in order to increase wealth.