Wharton professor Jeremy Siegel proved to be right on with his prediction about the Dow Jones Industrial Average hitting 18,000 by the end of 2014. Now, heading into 2015, Siegel says stocks will have a tougher go of it.

Siegel tells CNBC that stocks are getting closer to fair value than they have been in the last 6 years, meaning the easy gains are probably gone. “The last three, four years, I thought this was easy. I mean, it was a slam dunk,” he said. “The market was so undervalued with the interest rates so low, and earnings momentum going up. … Earnings momentum is going up, but we are closer to fair market value.”

Still, Siegel thinks stocks will remain below fair value for some time, in part because interest rates will remain permanently low. “I think we’re going to see Fed funds long run maybe 2 percent instead of 4 percent, and that lower interest rate is going to feed people to move into stocks,” he said.