Here's a good video explaining why Putin "might" invade Ukraine (as it came out about 3 weeks before it happened). It explains the situation going all the way back to the empire days.
dark0821 explained it pretty much above. They are worried about being next to a NATO country with no defense. So instead of saying "shit maybe we should make peace with those people so we can live harmoniously without actually having to worry about our border", they just invaded them.