Today the market is down on the usual fears from Europe. More importantly, the economic news is getting worse every day. Eventually, the market responds to fundamentals. The only question is when.
The world is afloat in cheap cash, and as long as that is so some of that cash will find its way to the relatively stronger American markets.
A permabull was just yammering about how the European stock markets are 40% undervalued to the American ones. He thought that this was a buy opportunity for Europe. Another way to look at this is that the American market needs to catch up to the European markets.