You want to be on the sidelines when that happens. Discipline and an unemotional approach to investing are keys to long term successful investing. Emotions get in the way of rational judgement. I can't tell you how many investors react emotionally to a move in a stock, only to regret it later. Investors jump on board stocks that have already gone up and dump fundamentally sound stocks that don't move fast enough.
Emotions are what get investors upset about movements in their portfolio from one month to the next.
A disciplined approach spreads the risks out among various asset classes. An unemotional approach realizes that everything isn't going to be going up at the same time. Balancing a portfolio gives a much better, long-term return. Today, many investors are in danger of falling victim to the next bear market. They have been blinded by the recent returns and have forgotten about risk management.