He said that any good investor always needs to ask themselves, "what could go wrong?" That holds especially true as the markets are surging higher.Cramer said the first thing that keeps him up at night is the looming crisis between Israel and Iran. He called the rising tensions the "elephant in the room," and not one to be taken lightly. With the European economy still slowing and the U.S. producing more oil than in the past decade, he said oil sanctions against Iran may not be enough. Similarly, Cramer's worried about the price of gasoline. He said that $4 per gallon has always been the breaking point for Americans, the point when there's simply not enough money left over for dining and shopping. "We need gas lower," he said simply. Third, Cramer said he's concerned with taxes going up. Federal Reserve chairman Ben Bernanke said taxes were a concern, and Cramer said if Bernanke is worried, then he is too. Fourth, Cramer said he's worried about all of the perma-bears, the naysayers for the last 6,500 Dow points, who are now turning bullish on the markets. When there's nobody left selling, noted Cramer, that's the time to worry. Finally, Cramer said he's worried about a sell-off on either good or bad news about the Greek debt situation. With the markets rising in the face of the continuing crisis, Cramer said he expects investors to use any resolution to take profits. But there are also some things Cramer said he's not worried about. Cramer said he's not worried about earnings, for one, as they're terrific. He said he's also not worried about politics. In a simple explanation, Cramer said if Obama wins, then things must be getting better, and if he doesn't, then our president will likely be someone the stock market likes better. Cramer noted he's also not worried about so-called "stretched valuations." He said that stocks are only expensive when looking through the prism of the past few years. Looking ahead, those valuations are not stretched in the least, he said.