NEW YORK (CNN/Money) - Brothers and sisters, not everyone believes in this rally.

They say that valuations have got too stretched. They say that investors have been blindered by bullishness, and so may be blindsided by bad news. They say the economy is still in sorry shape, and that at the end of the day all the efforts of the Fed and the White House will come to naught.

What are these naysayers, these pessimists, these heretics to do when confronted by the market's biggest and best rally in over three years? It depends on who they are.

For the individual investor, the answer is easy: Don't play along. Just keep on stuffing money into that mattress. Buy a little less, save a little more, try to not get too worked up about how cocktail banter is shifting away from mortgage refinancing to what price some guy says he bought Broadcom at.

For the hedge fund manager, the answer is easy: Don't play along -- too much. Get on board for a trade if you must, but keep your finger hovering over the sell button. Do your research and look for the stocks that have the stupidest valuations. Get ready to go short.

But for the mutual fund manager, the answer is hard, first because you can't short the market and second because you are judged against your peers. If you miss out on a big bear market rally -- and bear market rallies can last years -- you're not going to have a job anymore. Being proved right later on is small solace for not having made any money or gotten paid big bonuses to salt away for the lean times.

So you have to play along. You can complain about it, you can lose your hair over it, but there's nothing else you can do.

Now think of what this does to the market. First, realize that these bearish mutual fund managers were not as big in stocks when the rally began as their peers. Their relative performance looked good in March, but they lost pole position fairly quickly. Now they're struggling to catch up, and the only way they can do that is by throwing money at stocks -- especially the ones that they probably hate the most, like tech and biotech. And so the rally goes on.

In the end, what you end up with is a bunch of fund managers who, for fear of losing their jobs, have been pushed into extremely herd-like behavior. They all rush from one side of the boat to the other, and the result is extreme volatility. Huge moves up and, unfortunately, huge moves down. The next few months could be interesting.