After decades as some of the world’s most disappointing stocks, Japanese equities are suddenly lighting it up once again. The Nikkei 225 index has surged 28% this year through Monday, roughly 10 percentage points better than the S&P 500 index of U.S. stocks and more than double the gains of markets in South Korea, France and elsewhere around the world. Several reasons are driving the rally, which strategists at Goldman Sachs think can continue. One threat that could restrain Japanese stocks is the yen's possible strengthening against the dollar.