Russia and Ukraine both want perpetual access to Ukraine's reserves of coal (mostly in Donbass, under Russian control) and iron (mostly due north of Crimea, currently under Ukrainian control).
This is similar to the backstory of both world wars. Alsace Lorraine had coal and the Saarland had iron. These territories changed hands between Germany and France multiple times until Germany and France signed a treaty in 1951 to economically share them. They had to agree to keeping their borders open to trade in those goods and agree to keep their currency exchange rates relatively stable. These are the things Ukraine and Russia should be doing, and Ukraine joining the EU does the opposite. But Ukraine's stance is understandable. Russia is a corrupt dictatorship, while the EU upholds democratic rule and fair play. Russia needs to reform itself to be the kind of trade partner that would be attractive to Ukraine.

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