Its no secret, Putin wants to reconstitute the old Soviet Union and reestablish its power and influence. He’s already succeeded to a remarkable degree. Look at his actions in Syria, Iran, Egypt, etc. Putin is on the rise; we are on the decline (under Obama).
They still consider Crimea their port. They have a lot of gas pipelines running through Ukraine servicing Europe that probably account for a good chunk of the Russian budget.
Putin is not going to walk away and say “OK, you can have it”.