Another reason to rebalance is because you have too much risk in one stock. If you usually hold 10 stocks in your portfolio and balance the risks, then each stock should have a weight between 5% and 15%. If a stock that makes 10% of your portfolio doubles, then it would make 18% of your portfolio creating a larger exposure. Given the high proportion of risk in this one stock, it might be wise to trim the position since anything can happen, especially if such a large part of a portfolio is in one stock that has perhaps seen its stock price go up but has seen its fundamentals unchanged.
