It's very difficult for any particular segment of the stock market to sustain superior performance. The watch word for our financial markets is, "reversion to the mean" i.e. what goes up must come down, and it's true more often than you can imagine.
John C. BogleIf you have trouble imagining a 20% loss in the stock market, you shouldn't be in stocks.
John C. BogleThe grim irony of investing, then, is that we investors as a group not only don't get what we pay for, we get precisely what we don't pay for. So if we pay for nothing, we get everything.
John C. Bogle