Unforseen

longer time horizon means greater ability to live w/ the consequences of volatile stock returns. Think of a person nearing retirement in 2008 before the financial crisis hit. He's got a bunch of money saved up and as soon as he retires, he's going to buy an annuity to fund the rest of his retirement. If the market tanks and he loses 50% of his retirement in a few months, now he's got some problems.

If you're 30 years old and the market tanks, you're not as worried because you have a long time horizon to make that up. It hasn't really affected your future retirement if the market comes back eventually.