Broadly speaking, it’s because Putin is pissed that the Soviet Union fell (he lived through it as a KGB member in East Berlin), and Russia culturally originated around Kiev (https://en.wikipedia.org/wiki/Kievan_Rus%27), so he sees it as part of what should be the Russian empire. There are some important industries and resources in Ukraine, plus better access to the Black Sea. Ukraine’s orientation towards the EU and NATO is seen as an ideological and military threat. But the perceived humiliation of the fall of the Soviet Union, not being respected in the Western world, and trying to undo that and restore Russia’s glory, seems to be what is driving Putin the most.