Ah, that is a perennial question that befuddles many investors, especially ones who don't have any overall plan for their portfolios.
Many investors have only a vague asset allocation plan.
They invest in things that appeal to them at the time, usually based on something they saw or read or heard. Almost invariably, these investments have had stellar recent performance. If you're in that situation, you may feel as if you are facing a great dilemma. Get rid of the losers and keep the winners. And in order to sell high, you have to sell the winners.