After the Orange Revolution Ukraine started moving towards the West, approached European Union (lots of Ukrainians work in Poland and rest of EU) and started to fight with corruption.
This 'westernization' process started to slowly make Ukraine richer.
A rich Ukraine is an end of Putin's rule, because average Russians would realize that Russia could be a normal country too and they could be rich too, instead of living without toilets and being robbed by oligarchs. But in order to do that they would have to get rid of Putin and his clique.
So Putin tried to conquer Ukraine and when this failed, plan B is to ruin its economy.
This way Ukraine is as poor or poorer than Russia. This way Putin can secure his power.