Europe has, historically, had wars on a regular basis. There has never been a permanent settlement of borders, interests, or leadership.
It has nothing to do with peace treaties.
Wars in Europe are a normal phenomenon. Until 1945, when they stopped being normal. That was the Pax Americana at work. A remarkable achievement, that depended on the US guaranteeing the peace, and serving as the big stick of a like-minded alliance.
Ukraine has wanted to be its own thing, independent, intermittently since the days of Hetman Chmielnyki. It almost managed it in 1918-21. Other ex-Russian empire lands managed it then. Finland, Latvia, Estonia, Lithuania had never really been independent either, but they became independent at that time.
Ukraine has a perfect right to independence, no less than the Czechs, Poles, Finns, Balts and the rest. The Ukrainians voted for it overwhelmingly in 1991.
The Russians are, as usual, being dicks.
The Russians want a buffer against European invaders. Ukraine and Moldova and Belarus and Poland are that buffer