Inflation: Gasoline costs more today than it did last year. The increasing costs of goods can reduce your purchasing power over time.
Market Fluctuations: If the market moves down, you may need more time to recover. Owning different types of assets (read: diversification) can help limit the impact of volatility and allow you to have a more successful retirement.
Sequence of Returns: The order of positive and negative returns can have a huge impact on how long your savings will last once you start withdrawing money from your portfolio.