To everyone who is saying that Putin is a mindless thug, please read this.
Disclaimer- I am overwhelmingly in support of peaceful resolution of this conflict. I am just trying to list some reasons which many people are not giving enough though to.
Why Ukraine is so important for Russian security- Ukraine is kinda the gateway for invasion into Russian heartland. Launching an invasion from Ukraine would be very swift and easy because anything east to Ukraine is plains in Russia. Plains are ridiculously easy to move through, unlike mountains. Once you cross those plains to go to the other side, you would cut Russian heartland from Chechnya in South.
NATO and Ukraine- NATO is a military alliance made during the cold war to defend against the Soviets (now Russians). It's no secret that Ukrainians or atleast their political elite have been wanting to join NATO and EU since many years. More and more NATO extends towards the east, Russia gets more and more vulnerable. Belarus and Ukraine are the only IMPORTANT AF neighbours who haven't joined NATO yet. Ukraine joining NATO would be nail in the coffin for Russian security and leverage in the future by having the 'enemies' literally at their doorsteps.
The Future- It has been now or never situation for Putin/Russia since 2009 NATO declaration. Year after year, Ukraine gets better militarily, more compatible with the west and better prepared against Russians. Longer they stall the invasion or some kinda settlement, the worse off Russians are. Either you get something now or nothing in the future.
What about Putin wanting to be a badass and have popular support in country- Dude's been in power for 20 years