Why do people sell rising stocks quickly and hold on to falling ones for a long time?

This is called the disposition effect, the tendency to sell assets that have gained too early and to hold assets that have lost value too long.

It is explained as the combined result of wanting to avoid realizing losses, mental accounting that measures gains and losses against the purchase price, the desire to avoid regret, and self-control problems. Rather than using the purchase price as your yardstick, it is better to check whether the reasons for your investment are still valid now.