Surely when people invest in the market, they understand that it will fluctuate. Everyone wanted stocks during the dot-com bubble, and now nobody does, and like others have said that would be "Buy high, sell low". Doesn't work.
And as far as trying to guess when to jump in and out of the stock market, studies have proven that you'll probably not time it correctly. A 1998 story from Business Week showed that if someone invested $10,000 in 1988 and kept their money invested for 10 years, they would have ended up with $42,338. If by trying to jump in and out they missed only the six best months over those 10 years, they would have had just $25,402. That's 40% less money by missing only six months over 10 years.
There's two reasons for this. First, in the big picture the market goes up. Not every day, month, or year, but in the long term. Second, the market can go up a lot very quickly. So to be out of the market is to bet against the long-term trend.
Investing is not for everyone. But it is the best option for long-term financial gain. Accept the fact that there will be ups and downs, and as long as your retirement isn't right around the corner, just wait it out.