Veteran investor Jeremy Grantham thinks the artificial intelligence boom has pushed the U.S. stock market to its most expensive level ever and could eventually lead to a historic decline. "Based on the value of the stock market compared to GDP, with modifications, this is the most expensive market in American history," Grantham told CNBC's " Squawk Box ." While the GMO co-founder said he wasn't sure there was a comparable period, the tech bubble of 2000 is the closest analogy. He also