Everyone who's not living under a rock knows about the 'war' going on between Ukraine and Russia. People don't really know why Putin is invading Ukraine but there are a few probable causes:
The huge natural gas reserves on Ukrainian land
Crimea's water scarcity problem
Russia's poor birth-to-death ratio
Probability of Ukraine joining the NATO (North Atlantic Treaty Organization)
The gas pipeline in the centre of Ukraine
Russia, right now, supplies the more natural gas than any other country other than the US and is the 3rd-largest oil producer after USA and Saudi Arabia. EU also imports more than 40% of their natural gases form Russia. And it is the only petrostate inside Europe and it literally depends on gas for the economy with gas contributing about 12% of Russia's own GDP. So, it can't really afford competition.
But after technological advances in the 2010-era, Ukraine, believed to have about 1 trillion cubic feet of natural gas was revised to having 36 trillion cubic feet (shale gas) of gas (1% of total natural gas) and also included in that as 1 trillion cubic feet of gas off-shore in Crimea.
Note: Info on the stats of shale gas in Ukraine may be wrong.
After hearing Ukraine's gas in Crimea, Moscow, in 2014, invaded Crimea and annexed it claiming it was of historical importance. After the invasion, Russia had access to 80% of the off-shore shale gas deposits (almost 800 billion cubic feet).
But Crimea had only one problem -- water. It has only one source of clean and fresh drinking water, the North Crimean Canal, which the USSR had constructed hen Russia & Ukraine were one country along with 14 others. As the canal starts in Ukraine, they literally just SHUT IT DOWN. And since then the Crimean Peninsula has been becoming a wasteland day-by-day.
Russia since when, I don't know but it has the lowest birth-to-death ratio in the world. They're literally losing more people than giving birth to 'em and that's kinda bad... isn't it?
If Russia manages to invade Ukraine successfully, then, the numbers most probably'll go up.
Russia right now has to defend a plain (East European Plains) which is real huge and that means a lot of flat lands to defend, if NATO announces war against Russia, it's gonna have a tough time defending itself. Now, Ukraine is just that perfect barrier between all the NATO sates and Russia. But, if Ukraine joins NATO, it'll be a big problem for Russia, as the organization is bordering the country with thousands of kilometres of borderlands to attack from. So, if Russia invades Ukraine right now, it can easily tell Ukraine not to join NATO. Plus, Russia would have attacked it anyway and now is the best time for Russia -- when Ukraine hasn't yet joined the NATO as under its Article 5:
“The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.”
All this summarized means that if a country attacks a NATO member, all other members of the alliance should also consider it as an 'act of violence' and attack the opposing country. So, that means, tough time for Russia if it invades Ukraine after it joins the NATO.
When USSR existed, Russia was still a petrostate and it's largest customer, Europe wanted direct lines for gas. So, the USSR government made a pipeline that goes right in the middle of Ukraine. This line meant money and a lot of it ($$$$ type).
Now, Ukraine is charging a lot of money to Russia for the line to keep operating. And Russia is losing millions of dollars just because of the few the have to pay Ukraine for the line to keep on going. And even though Russia has now built an undersea line below the Crimean Peninsula and an underground line below loyal neighbour - Belarus, it'd be a good thing for the country if it could stop losing millions of dollars of revenue from the gases they export.
Well that's most of the major reasons for the war but that's about it.