If an average is unusually high/low it will tend to go back to the mean after time. In the stock market this means that looking over a short timespan one will

If an average is unusually high/low it will tend to go back to the mean after time. In the stock market this means that looking over a short timespan one will see large variance in the returns but over a long timespan (30+ years) the returns will be more consistent and nearly all positive.