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I think there are many reasons for this. First of all, stocks in the US exchanges are always valued higher than stocks in other exchanges. Second, as the boomers retire, more money is being pulled from their more risky (overseas) investments and brought back to the US. Third, as other markets collapse (China, Russia, etc), or struggle (Germany, Australia, UK) that money is also being brought to the US instead. Fourth, the US dollar is very strong at the moment, which makes everything priced in dollars have a higher value and attracts even more foreign money to the US markets.
Does all this mean there won't be a correction? No, I suspect a correction is coming, and I'm getting out of US assets. In fact, I'm starting to bet against some US assets, especially "green" tech.
Does this mean that valuations will come down to be on par with valuations in foreign markets? Probably not. I think the US economy and asset market are going to outpace most foreign markets for the next few decades. The few exceptions will probably be India, Vietnam, and maybe New Zealand and Argentina. Personally, I have my entire Norwegian pension fund bouncing back and forth between the Norwegian and Indian markets, and I'm selling US assets to move them to Vietnam.