...This is a function of the carry trade, in which speculators borrow money in near-zero interest-rate yen and buy U.S. stocks with the cash. The financiers make money in two ways: the buying pushes the U.S. stocks up and the decline of the yen means they can pay back their loan in cheaper yen.
...As long as corporations continue borrowing money to buy back their own stocks and the yen keeps dropping, the SPX will continue lofting higher. If either of these drivers fades or reverses, the rally in SPX will reverse, too.