Others have made brilliant points. To expand on them: Ukraine holds a special place in Russian history: Peter I, and Catherine II both took key parts of what is now Ukrainian territory. Peter I got Kyiv, and Catherine II's forces annexed Crimea and definitely everything East of the Dnieper (I can't remember exactly how much land, sorry!)It makes you wonder what will happen if they do take over the Ukraine. What have they actually ended up with? A country that isn’t internationally recognised by many, a population that doesn’t respect its leaders, a country that’s been destroyed that needs millions of pounds to repair , economic sanctions still being held thought the west. It makes absolutely so little sense.
Putin wants to be remembered in Russian history as one of the great leaders, and this could, to his mind, be a way of gaining some of their 'glory'/ popularity. Capturing historically Imperial Russian land might be his way of cementing this reputation.
Stepping back in time. Peter I also tried to wage war on Sweden (modern day Estonia and Latvia), and and Catherine II gained a lot of what's now modern day Poland as territory by supporting Stanislaw III. Lots of countries that are established free democracies might be seen Putin the Whacked as de-Russified land that belongs to Russia. When he was growing up, these countries were Soviet states so it's not a massive jump for him to still view them in that way.
It's been posited that capturing Ukraine is just the start, and it's not improbable that he's going to try picking off the non-NATO countries one by one.