Why must the opportunity cost of a decision always be something desirable?

The opportunity cost of a decision refers to the value of the next best alternative forgone when making a choice. In other words, it is the benefit or value that an individual gives up or sacrifices in order to pursue a particular course of action.

The reason why the opportunity cost is typically something desirable is because individuals generally choose the option that provides them with the greatest benefit or utility. When making a decision, people evaluate the various alternatives and weigh the costs and benefits associated with each option. They ultimately choose the option that they perceive as offering the highest value or greatest satisfaction.

For example, imagine you have $100 and you are considering spending it either on a concert ticket or a new video game. If you choose to spend the money on the concert ticket, the opportunity cost would be the satisfaction or enjoyment you would have gained from playing the video game. In this case, the opportunity cost represents something desirable, because you would have preferred playing the video game but gave it up for the concert experience.

However, it's worth noting that not all opportunity costs are inherently desirable. Sometimes, individuals have to make choices where all available options have negative or undesirable aspects. In such cases, the opportunity cost may refer to the option that is deemed the least undesirable.

opportunity cost is the value of the next best thing when making a decision.

Say you have a decision to buy A or B. You know the value of each decision. You choose A, and it cost you the opportunity to choose B.

I can't answer your question, mainly because of the word "desirable". That is not an operationally objective word, what is desirable for one is not desirable to another.