For example, someone in their 30s who is investing for retirement can afford to invest through up and down market cycles because they have a long time for their money to work for them. An investor closer to retirement doesn't have as much time to recover from either mistakes or a down stock market. People that invest at the top of markets are looking in the rear-view mirror. They've watched a stock, for example, that has had great gains for the year and they feel safe getting into it. The investor that's in the stock is getting ready at that point to sell it.
People tend to do the opposite of what rationally makes sense.
Everyone knows you want to buy low and sell high, but investors tend to do the exact opposite. Part of it is an education process. We like to use a dollar-cost averaging strategy to purchase more shares when prices are low, and fewer shares when prices are high. As a practice we prefer dollar-cost averaging to smooth the bumps out a little bit, and it usually gives investors a little more comfort level. But we will lump-sum invest if we feel the market is at or near its lows.