Because most of the people living there are ethnic Russians and want to be part of Russia. No, this is not "all Putin's fault". The US has been escalating the tension there for decades by building up NATO further and further east (after lying and saying we wouldn't) and then we backed a coup which overthrew their president in 2014, and installed a puppet government that is beholden to US interests. Zelensky is an actor who seems to have been groomed for the role of Ukrainian president, and he's putting on his greatest performance yet - that he's actually a leader of a sovereign "democratic" nation and not a US intelligence asset leading a corrupt government in cahoots with extremists. Ukraine is a mess, and the poorest country in Europe. Russia could actually just roll right over them if they wanted to, so that's obviously not what this is really about.
It's like if Russia overthrew the Canadian prime minister, picked the new prime minister and set up bases close to the border with the US. Obviously the US would probably attack or be close to doing so. That's essentially what's going on here.