Ukraine may be a young state, but the ethnic identity is considerably older. And yes, of course there are significant cultural ties due to shared origin - so what? Serbs and Croats are even closer, which did not preclude them from trying to ethnically cleanse each other in Bosnia.
> it was happy to be part of Russia/USSR
It was so happy in Imperial Russia that the authorities saw it fit to severely censor publications in Ukrainian:
Most large nations have these kind of issues from time to time. US had the civil war. Still, I wouldn't say that historically Ukraine wanted to be completely separate from Russia in the same way as Ireland has (from UK).
Nationalists are usually the loudest minority, while the bulk of population does not really care.
The only time I can think of when Ukraine as a polity expressed the desire to be aligned with Russia was the Pereyaslav Rada during the Hetmanate. And that one was really a choice between Russia and Poland, so the only question they answered is whether Ukraine is culturally closer to one or the other (no surprise there, given their religion). It did not preclude the subsequent hetmans from seeking foreign aid against Russia in further rebellions.
> it was happy to be part of Russia/USSR
It was so happy in Imperial Russia that the authorities saw it fit to severely censor publications in Ukrainian:
https://en.wikipedia.org/wiki/Ems_Ukaz
It can also be instructive to look at this list and see how many of those were against Russia.
https://en.wikipedia.org/wiki/Cossack_uprisings
As for the USSR, well, there was that whole Holodomor thing.