there probably is more than one reason to do what russia is doing. i do tend to believe their claim to make ukraine abandon their nato aspirations, and there probably are more reasons, like, for example, the persistant corruption scheme with russian gas that has made ukrainian oligatchs millions. funny how the start of the war coincided with completion deadline for northstream pipelines 

 There are many reasons, ranging from purely historical ones, such as Kiev being literally the cradle of the Russian world, to economic and military factors.
https://www.youtube.com/watch?v=IsjKZstp-XU&t=81