Traders work on the floor at the closing bell of the Dow Industrial Average at the New York Stock Exchange on January 10, 2019 in New York.

The markets are starting off with a somewhat defensive edge on Tuesday, but the overall trend has been remarkably positive. So, why are so many people cautious about the markets?

You wouldn't be cautious looking at the details. Breadth as measured by the advance/decline line has been improving almost every day since the Dec. 24 bottom. It means more stocks are advancing than declining, and it's the single most important indicator I look at. That A/D line is now essentially near the historic high it hit late in 2018.

Other internals are also strong: New highs are expanding, selling pressure is low and the Dow Jones Industrial Average is riding an eight-week win streak.

The market leadership is tilted heavily toward cyclical stocks, which market bulls always like to see lead. Since the Dec. 24 low, industrials are up 25.3 percent, energy stocks are up 22 percent, technology stocks are up 20.6 percent, consumer discretionary stocks are up 19.6 percent and financials are up 18.2 percent, according to Oppenheimer.

At the same time, the laggards have all been defensive sectors. Health care is up 14.9 percent, consumer staples are up 11.6 percent and utilities are up 7.6 percent, Oppenheimer noted.

Another positive is that this is not solely a U.S.-led rally. There is a modest global breakout going on as well. Europe's Stoxx 600 is at four-month highs, as are markets in Shanghai and Korea. Japan is at two-month highs, and oil is at a three-month high.

Yet so many strategists are sounding cautious, like John Stoltzfus at Oppenheimer, who told clients Tuesday morning, "We believe investors should leave the party hats in the box, stay focused on their goals and objectives and keep expectations right-sized avoiding complacency."

It's mostly because the good news, such as progress on trade talks, the Fed putting rate hikes on hold and strong market momentum, has been offset by clear signs of slower global growth (Europe and China especially but also possibly including the U.S., where last week's industrial production and retail sales numbers were especially disappointing) and U.S. earnings expectations that are near zero for the year.

Still, traders like NYSE floor veteran Peter Tuchman of QMS Directex are still taking in the extent of the rally and believe there may be more to come: "No one expected a fast and furious rally coming out of that Christmas Eve sell-off. Now all of a sudden we are up almost 20 percent, and everyone has a list of stocks they want to buy and the market is not helping them. A lot of people who got out during the market downturn in December are now notably underperforming the markets."