It was originally part of the USSR. Putin has said many times he thinks breaking up the USSR was a huge mistake and wants to re-acquire those territories.
But yeah why does anyone start a war? Power, control, money, distraction, madness, idealism, etc. Probably a little of everything.
Putin said in his recent speech that he thinks Lenin made a mistake in breaking up the Russian Empire into independent nations (that then joined the USSR). I think Putin is more after restoring the old Russian Empire than restoring the USSR.
But yeah why does anyone start a war? Power, control, money, distraction, madness, idealism, etc. Probably a little of everything.