A. By building a diversified portfolio, you are admitting you can't predict the future, because otherwise you will invest all your money in the asset class with the best potential to appreciate.
Because we don't know which asset class will outperform in the future, it's important to periodically rebalance, so you don't face the risk of outsized loss from the outperforming asset classes, because there is always a time for the asset class to fall, the problem is we don't know when that is going to happen.
In short, rebalancing is the logical way to buy low and sell high. Studies of past stock market data also support this approach (when comparing a diversified portfolio rebalancing once a year versus not rebalancing at all).