Most people don't realize that you lose a lot of money when the market is trending horizontally, like it has in the last decade. For example, assuming you have 10,000 invested (makes it easy), if the market goes down 50% today and goes back up 50% (extreme example), what will be your net profit or loss? Most people think they will be back at 10k, for a net loss of 0, but that isn't the case.