People demand more to give up a thing than they would pay to acquire the same thing. Ownership itself adds perceived value.
In a portfolio this becomes an unwillingness to sell. Ask yourself the honest test question: if I were flat right now, would I buy this here, at this price, in this size? If the answer is no, you are holding it because it is yours, not because it is good. The effect is strongest on positions with a story attached, such as a first big winner or a company you researched for weeks.
A simple counter is the flat test above, applied at a fixed time each week to every open position. It converts holding from a default into a decision.
Related: status-quo-bias, disposition-effect