Noted bull Tom Lee is calling for double-digit gains for the S&P 500 next year, but told CNBC investors are going to have to get used to an uncomfortable bull market.
"We're getting into the realm of where investors really have to start embracing the ability of the market to rerate," the founder and head of research at Fundstrat Global Advisors said in an interview with "Squawk on the Street."
In other words, people only expect the market to go up on corporate earnings because the price-earnings ratio is at its long-term average.
However, Lee believes P/E expansion will continue because "as you move and mature in a bull market, P/E expansion is what takes root."
"We are going to be surprised. It is going to be an uncomfortable bull market but people will need to get used to that."