Every nation has the right to self-determination for its own state. In Kosovo they had a referendum and decided for their own state, like Slovenes. They can't join e.g. Albania, because it's not the same and internationally no one would recognize them in that case. That's why the Serbian president doesn't support what Putin wants, because then he might have to give up Kosovo and Chinese would have to give up Taiwan. Interestingly, those who support them have different problems.
In Ukraine they had a 1991 referendum and decided for their own state and seceded with 92%. In Donbas and Luhansk it was I think 80%. In 2014 Russians with separatist help (they supplied them with heavy weapons and sent mercenaries) occupied those regions and Crimea. That east never wanted to be under Russia but wanted to be more connected with Russians than with the West. Otherwise it's more because money comes to those places from Russian oligarchs who own factories. Putin wants to occupy those 4 regions and annex to Russia, like much else.
E.g. just like you adore the Russian system and it would be better if you move to Russia. Not learn two words of Russian and with your like-minded (separatists) sow discord in Slovenia and Putin would send you help.