Despite the U.S. stock market's continued rally, investors are becoming increasingly more pessimistic.

The AAII Index registered 33% bulls and 45% bears, less bulls and more bears as the market goes higher and higher.

Barron's was also filled with negative sentiment. Steve Leuthold, of Leuthold Group, had little positive to day about the market. In addition, Barron's back-page editorial wrote of the structure bear market and high inflation of the 1970s. Further, even this week's book reviews spread negative vibes. Bubblicious, a history of bubble mania (which sounds like an excellent read), wrote of wildly over extended markets.

Mike Santoli titled his piece "The Bull Battle Fatigue," evidence of which is hard to find since the bull market is showing little sign of ending its upward ascent.

Despite a very good market for 2007, few want to suggest that this is simply a good and healthy bull market. Every rally leads to more and more pessimism.

Wait for the classic sentiment indicates to show too much optimism before becoming negative on the market.