David Rosenberg, chief economist and market strategist at Gluskin Sheff, famously (and for some, infamously) turned from a long-term bear to bull in 2012. He's still optimistic about the U.S. economy and American equities. But now, Rosenberg admits to becoming somewhat discouraged by data he's seen—and perhaps a bit more skeptical about the future.

"I have to say that for a former bear-turned-bull who tried for the past two years to see the world from a cup-half-full lens, the last few weeks have been more than just a bit frustrating," Rosenberg wrote in a Monday note. While he's not changing his views just yet, Rosenberg said he has "no intention of sitting on a stale view," and compared his reassessment to a ratings agency putting "a doubtful debtor on 'credit watch.'"