What's China's Need For A Buffer State?

Common wisdom holds that China should be expected to assist America in bringing to heel a rambunctious and militaristic North Korea.  Trouble on their border, everyone understood, wouldn't serve China well.

But, generally, no asks why China needs a buffer state in the first place?  If it doesn't want Chinese leaving China without approval, or if it doesn't want outsiders coming into China without Chinese approval, wouldn't a fence serve just as well.  It seems to work pretty well for Israel.  And, of course, it's what Trump is planning to build for America.  Does China really need a nation on its border that treats its citizens like so many serfs laboring to buff the image of its fat, little, child-like leader; a leader that thinks nothing of using an artillery weapon to murder his uncle and others among his circle of friends?

After losing it's union of nations, Russia now finds itself surrounded by nations like Finland, Latvia, Estonia, Poland, etc.  It's relationships with these countries are as good or as bad as Russia chooses to make them.  The only real problem for Russia is that it has aspirations of empire.  It's really a sorry country; huge in land mass, but with a population inadequate for its size.  It's love for a centrally controlled economy dooms it to industrial inefficiency and kleptocracy.  It's main export is oil and gas.  Indeed, energy exports are it's only real ace-in-the-hole.  That, and its propensity for making trouble for its neighbors, e.g. Georgia, Crimea and the Ukraine.

But what's with China?  They have a substantial land mass.  They've got a sizable population.  Their most peaceful border seems to be with Russia.  (I guess the two are fairly well matched.)  But China can't seem to make peace with India.  Mianmar, Laos, and Vietnam are too small and underdeveloped to give China any sort of trouble.  And, yet even with it's small and scrawny neighbors to the south China can't seem to live in peace.

What is China thinking when it declares that the entire China Sea belongs to it and it alone.  Little, uninhabited islands that have been acknowledged for the longest time as belonging to Thailand, the Vietnamese and the Philippines, along with China, are all claimed by China.  They have taken  uninhabited islands and have built them up to accommodate military bases.  Further, they have denied fishing rights to the nations that have exercised such rights for generations.  If China were to attempt to restrict free movement through the China sea it would mean war.  What is China's problem?

I guess my problem is that I'm an American.  I see a peaceful neighbor to the north.  To the south there are problems; two in particular.  It seems to be our border with Mexico across which most narcotics flow into America.  Second, it is this border across which millions of migrants have flowed with absolutely no control on the part of our government.  Hopefully, these problems with Mexico can ultimately be resolved by better policing.  It hardly seems something that requires aircraft carriers or atomic weapons.

Our real problem is with China rather then North Korea.