Profit should be realised from sales of stocks in the following situations:
(I) when the stock is obviously overpriced, or
(II) when the sale of the stock frees the capital to be reinvested into another stock with potentially better return.
Not taking profit in the above situations can harm your portfolio and compromise its returns. In other circumstances, let the winners run.
Underperforming stocks should also be sold early. Hanging onto underperforming stocks is costly too. There is the opportunity cost that the capital can be better employed for higher return. Also, hanging onto these lack-lustre stocks reduces the overall return of your portfolio.
Reducing serious loss
When the fundamentals of a stock have deteriorated, sell to protect your portfolio. This decision should be make quickly based on the facts and situations, in order to keep your losses small.