From the New York Times:
Bad news is cheered. Good news makes investors nervous. Welcome to Wall Street.
The S&P 500 rose above 3,000 for the first time in its history on Wednesday, with gains that continued to push the index toward a record on Thursday.
The most recent jump began after Federal Reserve chair, Jerome Powell, suggested the nation's central bank was worried about the economy. Just days earlier, strong data on the job market had the opposite effect for stocks.
This counterintuitive reaction to the news is a phenomenon that's explained by expectations for interest rates. The weakening outlook for the economy means, in all likelihood, borrowing costs are coming down – and in the right circumstances, this can be good for stocks.
If that all sounds familiar, there is good reason. Those same conditions were in place for much of 2012 to 2015, when the S&P 500 rose nearly 45%.