Why would the Russian government decide on such an extreme measure as a large scale attack on Ukraine?
Russian vision of "Slavic Unity"
Both the Russian and the Belarus Presidents have recently stated that they believe that the 3 Eastern Slavic nations of Russia, Belarus and Ukraine have such deep common roots that they should unite into one common state. Historically, this is true roughly for the last 3 centuries of the Russian Empire.
It has to be taken into account, though, that the Belarusian and Ukrainian people were usually not treated as equals by the Russians, with the rural population of both nations speaking their original languages and only the imported elites speaking Russian. One result of this disdain is the "Holodomor", the mass starvation of Ukrainians under Stalin, which is in a way compareable with the treatment of the Irish under English rule during the Potato Famine.
Russian disrespect for existing borders
When the USSR broke down, the principle was to accept the Republics' borders without changes as it was clear that any possibility of border changes would incite conflicts. Early on, that broke down, with the Mountaineous Karabakh conflict between Armenia and Azerbaijan and the Transnistria conflict with Moldova, where a thin slice of the country became a Russian puppet state.
Those two conflicts were the direct result of the USSR breakup and as such seen as "sleeping". But Russia, in 2008, attacked Georgia and occupied South Ossetia and Abkhasia, a de facto annexation which the FM and especially Capricornia protested against at the time, seeing the future consequences.
In 2014, with the official annexation of Crimea, Russia granted itself the right to unilaterally change post-Soviet borders. The Crimean population might or might not have voted for joining Russia in a democratic referendum without occupation forces present - this cannot be decided anymore. But it is completely clear that Russia would never accept one of its republics or territorial units (say Chechnya or Tatarstan) to leave the Russian Federation, based on a democratic referendum. The conclusion is that Russia operates as an imperialist state, actually the last of the imperialist states with an expansive agenda, except China.
Both Putin and Lukashenka are getting older - looking at Kazakhstan, they understand that the probability of a peaceful pensioner time after giving up the presidency is not that high. Also, in October 2022, Putin will turn 70. Putin has made it public that the "Slavic Unity" is important to him, implying he wants to achieve this before the end of his Presidency.
Russian fear of democratic development
Russia has the typical characteristics of a Mafia State - an executive ruling without any democratic control, a legal system which is controlled by the executive and an elite which enriches itself using that basis to extort or even take over legitimate businesses. A typical element of the Mafia and of Mafia states is "omertà", the way elite members keep quiet about their internal struggles and organisation.
Unluckily, the efficiency of modern capitalism and technology have enabled modern Mafia states to survive for extended periods. Other typical Mafia states are the PR China, Belarus, many other states in Asia and Africa, and to a certain level also countries like Hungary, Poland, Turkey and, unluckily, Ukraine. Elements of Mafia states are present in many "Western" states, too, but usually in a reasonable equilibrium with democracy.
Ukraine has been working to develop towards this equilibrium, with openness towards the EU and a slow process to reduce the influence of oligarchs. Due to the cultural and geographic vicinity of Ukraine and Russia, this is highly dangerous for the Russian Mafia elites, as ordinary Russians would recognize an alternative to their system. This process could be compared with the democratisation of Portugal and Spain and even with the breakdown of Communist rule in Eastern Europe. Russia has developed methods to suppress this process as far as possible, but the elites know these methods might lose power at some point. Therefore, Russia and Belarus need to stop Ukraine from becoming a successful example of a democratic nation.
Time running out
After 7 years of warfare on the Donbass front, Ukraine has developed into a more united and nationalist nation than it ever was in history. Its military is being rapidly modernized. The arms industry has regrown from its post-Soviet breakdown and is now able to produce and export modern weapons. The cooperation with Turkey gives both sides access to industrial abilites and techonologies of the partner, as they share some strategic interests. It is possible that an Azerbaijan-Turkey-Ukraine regional alliance might be born in the near future.
One of the symbols of Turkish-Ukrainian cooperation are the Bayraktar drones, which have been used in the Armenian-Aserbeidjan war with deadly success. Drone technology is developing rapidly on all sides, but it is most threatening to armies relying on masses of tanks and armed vehicles. Similarly, anti ship missile technology works against states operating huge navies, like Russia. On a decades timescale, it is a "use it or lose it" situation for traditional armies.
Another factor is that Ukraine is stabilizing politically. To everyone's surprise, Zelenskij appears to be a competent President, able to "play a strategic game" by calming the Ukrainian people in this time of crisis. Many Western countries have decided to earnestly support Ukraine, especially its geographic neighbours.
In Europe, the pressure to reduce the inflow of Russian oil and gas to Europe is mounting, not only because of the climate crisis. That would reduce Russia's financial power and create internal conflicts.
Those developments operate on a longer term timescale. On a short term timescale, the timing of an Ukraine attack depends on weather (preferring frozen ground instead of swamps), the ability to blackmail the West (gas supplies), food supplies (avoiding harvest times) and some level of surprise. All these come together by beginning of February, ending by beginning of March / April, for this year. The main weakness of Ukraine, the lack of modern air defense, may be solved within one year with U.S. support, while Ukraine would also continue to acquire Bayraktar drones and other modern weapons.