My first day in business school I learned about opportunity cost. It blew my mind. (I was young.)
The opportunity cost of a choice is the value of the best alternative forgone, in a situation in which a choice needs to be made between several mutually exclusive alternatives given limited resources.
Anytime you use a resource (time, money, energy) for one purpose, you are choosing not to use the same resource for any other purpose.
This is a concept far too few people (Mormons and not) understand. When they say yes to something, they are saying no to a bunch of other somethings. I’ll get back to this later.