The Low Volume

By this time it's not secret that the stock market is trading at abnormally low volumes.

S&P 500 All Time Monthly Chart

As you can see since the last financial crisis volume has been decreasing while the market has been rallying. Moreover, current volume about the same as it was in the late 90's. I see three reasons for the following:

1. Investors are still fearful of the stock market since the last financial crisis along with the current euro crisis.
2. Baby boomers are retiring shifting their money from stocks to fixed income and safer assets. 
3. Long term investors are frustrated with how the stock market has not gone anywhere in the last 12 years.

As you can see though from the lines I drew, since 09 the market has been rallying while volume has been decreasing. I would point to the federal reserves actions to explain this (QE1, QE2, Operation Twist). I personally think that this makes this current uptrend very fragile, and a red flag with the low volume.