It's complicated because you hear so many different stories and versions as to why. My take is they regard Ukraine as basically Russian land and put up with Ukraine being independent when they had a pro Russian government but got annoyed when the Ukrainians, with American help, overthrew that and went western democracy.
But it's also part of a long pattern - over the last 500 years Russia has expanded at an average rate of 50 square miles per day by invading other people's countries and the like - it what they've always done. I'm not sure if that isn't a bit out of date these days - the whole imperial invasion thing. I mean I'm a Brit and we used to do it and had the world's largest empire but it's all a bit passe now. (50sq mile link https://www.foreignaffairs.com/articles/ukraine/2016-04-18/r...)
By the way re the history, Ukraine, governed from Kyiv is basically the same as Kievan Rus, also similar land governed from Kyiv and existed long before Moscow or Russia were invented. Muscovy, the bunch out of Moscow, were a warlike lot and stole the Rus bit of the name and much of the land. So when you here Putin going on about it not being a real country and just a bit of spare land belonging to Russia, it's not quite true.
But it's also part of a long pattern - over the last 500 years Russia has expanded at an average rate of 50 square miles per day by invading other people's countries and the like - it what they've always done. I'm not sure if that isn't a bit out of date these days - the whole imperial invasion thing. I mean I'm a Brit and we used to do it and had the world's largest empire but it's all a bit passe now. (50sq mile link https://www.foreignaffairs.com/articles/ukraine/2016-04-18/r...)
By the way re the history, Ukraine, governed from Kyiv is basically the same as Kievan Rus, also similar land governed from Kyiv and existed long before Moscow or Russia were invented. Muscovy, the bunch out of Moscow, were a warlike lot and stole the Rus bit of the name and much of the land. So when you here Putin going on about it not being a real country and just a bit of spare land belonging to Russia, it's not quite true.