Sound Mind Investing has a concise explanation of why contrarian investing works.
Simply put, the 200-year track record of the chart says that stocks are likely to produce better returns than their historical long-term average until they "catch up" to the trend line. Maybe not this year, or next, or for the next 5-10 years even. But time after time those two lines have separated and then converged, and it's likely to happen again before too long. It could take a decade, but long-term investors have time on their side.
I recognize that this is difficult to accept for many people who look at the long-term challenges facing our economy and our country. But keep in mind that all of the problems we see are already known and factored into the stock market's current valuation. The stock market is a forward-looking discounting mechanism that has all that known bad news already baked in.
That forward-looking discounting, coupled with the tendency shown in the chart for the market to revert to the mean, causes the market to continually deliver the exact opposite of what most investors expect. That's why in hindsight, a time like 1999 and early 2000 can be a poor time to invest, despite the fact that the external conditions seem to look great. And it's also why hindsight may well show the current period to be a good time for long-term investors to invest, despite external conditions seeming to look poor.