On the off chance you actually want a detailed analysis, here you go.
"Why Russia is doing this": It depends on how you define "this".
- If you mean "why is Russia trying to take control over Ukraine", then there's... like a dozen good reasons. Economic reasons mostly, some political, some ideological. At least one humanitarian. Most of them, however, have to do with the one thing the Russian government keeps telling the West that the West always says is stupid: the West has put too much pressure on Russia. Forget missiles and militaries for the moment, those functionally do not matter when it comes to Russia vs NATO, because nukes exist. Russia is not existentially threatened by NATO missiles, because even in the event that NATO launches missiles from extremely closely positioned sites, Russia will still be able to deliver a counter-strike that will be absolutely devastating to everyone. So the missiles that Putin alludes to in his speeches, those are a front. The actual problem, is the extent to which the West has been readying to deliver an economic and political blow to Russia, the extent to which the US can control NATO's so-called member states, and the extent to which it can control the public perception of Russia to suit its own needs. The practical outcome of this happening, would be a slow winding decline as Russia is gradually denied more and more of its economy base, and prevented from gaining any kind of power. There is only one solution to this - Russia needs to drop its reliance on the West, and assert a stronger position in the world.
Problem with that - Russia can't exactly just drop out of world trade. Internally, the people wouldn't support that kind of move, and the entire concept is too complex to be explainable to regular people. Because it'd mean a plummeting currency, denial of luxury goods, overall degradation of quality of life that a lot of the people got used to. So something different is needed, that people can rally around. Joke all you want, but even a "totalitarian regime" like we have here, wouldn't stand after directly making such an unpopular move.
Thankfully, the US is stupid and greedy. And they've decided that the Ukraine makes for an excellent new place d'arme to undermine Russia from, by cutting off their access to the Black Sea naval base at Sevastopol, and putting a new NATO or EU (or both) aligned country on the Russian border. The animosity between Ukrainians and "Moskali" is long-standing, and it wouldn't take much to organize the local extremists into putting the country onto a decidedly anti-russian course.
That's where this all began. Russia took action, preventing the loss of the Crimean peninsula and the Sevastopol naval base. The majority of the population being aligned with Russia made the transition relatively painless, but obviously neither the US or Ukraine liked that. Donetsk and Luhansk went into revolt as well, which Russia happily supported, and which US and Ukraine liked even less. Russia was gaining a foothold.
Fast forward eight years. The US has been stuffing the nation full of weapons and providing them with all kinds of expertise and connections to prepare for beating back the Russian 'occupation'. All attempts at peace were mocked viciously - even if Ukraine would have benefited from it, officially dumping occupied territories would have allowed it to fast-track into the EU or NATO since it wouldn't be having active hostilities anymore, but - well, the US didn't need peace in the region. It had weapons to sell, a NATO to strengthen, and a Russia to put in their place. For Russia, the people of DNR and LNR suffering under Ukrainian attacks, and the people of Crimea facing a rapidly approaching humanitarian crisis over lack of drinkable water, it's easy to rally people around a new goal. Ukraine, in the eyes of most of the public, has long been known to have Nazis among its military units. That notion has been carefully cultivated for eight years, likely blowing it far out of proportion - but it makes for an excellent, and painfully sore point for Russians, that is very easy to latch onto. Combined with efforts to help the besieged people's republics, it creates an excellent internal pretext for plunging Russia into the world of pain it has to go through to come out stronger on the other side. The sanctions, when they inevitably come, work against the West then - as they're treated as something that's being done to the Russian people from beyond the border, directing the people's ire more at the nations doing the sanctioning - and their perceived (and probably actual) US manipulators, than at the Russian government, which is "only doing what it must".
Why now?: Contrary to popular, apparently, belief, Russia isn't blind. Everyone keeps talking of Russian spies and saboteurs whenever something happens, but somehow they don't exist when it comes to keeping tabs on neighboring countries. This is a rumor, since it comes from what can well be our propaganda: Apparently, a military op was being planned in Ukraine for later in March, to take back DNR and LNR. If this is true, and were allowed to happen, it would have put Russia into a reactionary position. With the US controlling the public perception of events, and having leverage over most of Europe, Russia would have no way to justify preparing a defensive force to protect the regions, so it was either blatantly assemble an army on the border (behind a paper-thin veneer of exercises and maneuvers, which did actually occur), or lose the only possible pretext for moving forces into Ukraine territory and disrupt US plans to cripple Russia.
Bonus round: Why is Russia even attacking the whole of Ukraine, rather than just staying in the regions?: That would be because Ukraine is stuffed full of American weapons and combat-trained personnel, so merely reinforcing the border of DNR and LNR would still result in a war - just a disadvantageous one, where the enemy is free to fire indiscriminately into cities and can bring air support with impunity. Striking at as many military infrastructure targets as possible, reduces the Ukrainian army's ability to counterattack.
But why is Russia firing on all those cities, dropping bombs on civilians? Why are they leaving behind their tanks and MLRS batteries? Why are their troops so poorly equipped and useless?: Heh. Are they? Ukraine is backed by the US. The US has unmatched control over public perception of events. They control information across the whole world, and can not only pick and choose what to show, but can easily misdirect and deceive anyone in the West, because nobody there can seriously tell one eastern-european city from another, and both sides are using the same damn equipment.
Mind, I'm not saying all of what you're shown is a lie. A lot of what we're being told by our media and military is lies too. We do take casualties, we do have equipment break down on us, and it's almost guaranteed that at least some of the shells and missiles we fire land where they weren't supposed to. But do keep in mind, that almost the entire point of this whole operation, from the perspective of the US, is to discredit and depower Russia. And thinking that they wouldn't lie out their ass when they can absolutely get away with it, is at the very least monumentally naive.
(it's past midnight and I've barely proofread this; sorry for any typos)