Answer: Investors believe future profits will be higher than previously expected.
Step-by-step explanation:
If the S&P 500 has been rising, this means that investors are buying more shares in the companies in the index.
This means that these investors believe that the profits to come to these companies is going to be higher than expected. If the profits are expected to be the same as previously thought then there would be little increase in stock prices because the relevant increases would have already occurred.