Getty Images / Spencer PlattEconomist David Rosenberg warns the stock market is in a "mega-bubble."Rosenberg cites high valuations, investor positioning, and sentiment as warning signs.Despite recent market gains, Rosenberg predicts a recession and advises caution in investing.It's a tempting time to jump into the stock market. The S&P 500 continues to register new highs and has climbed an incredible 23% so far this year.