Here is the allegory: You are walking through Central park one day and you come upon a bum sitting on a park bench. The bum says to you "Hey, sir or madame, I am homeless and unemployed. My liabilities greatly outstrip my assets. I need $1 million to have my teeth fixed and to take a vacation. Can you lend me the money?"
Would you lend this guy the money? Now... imagine you are the Chinese Minister of Finance and the bum is Uncle Sam...