Opportunity cost
When I do something, I am also not doing something else. That sounds obvious, but I think it has to make the doing intentional.
It is easy to do what gives immediate pleasure right now. One evening of that means nothing. But as a habit, repeated over days and weeks, it means I keep missing the other thing I could have done instead.
What I do daily matters more than my goals. Goals state what I want. Daily systems and mechanisms decide what I actually do. This is where opportunity cost lives. I have to make my mechanisms strict enough that I stick to the goal I set, instead of drifting into side quests that are not the main question of my life. Every drift has a price: the opportunity I missed.
The compounding effect is real here. Over a few days the difference is hard to notice. Over years, the gap between the version of me that did what I set out to do at the start of the year and the version that did whatever was convenient at each moment becomes large. That gap can work in my favor or it can haunt me, depending on which side I chose.
The examples are everywhere. Staying in a good job instead of switching has an opportunity cost outside it. Starting a relationship early has one cost, starting late has another. I went to play badminton, which means I did not go dancing.
But life cannot be lived by tracking this every minute. When I do something, I also have to be present in it. I cannot always think about what I am missing. Like most things, there has to be a balance. I enjoy what I am doing, while staying mindful that I could be doing something else.
It is not only that whatever I do has to be done intentionally. The choice of what to do has to be intentional too. Choosing and doing are two separate acts, and both need intent. This is close to what I wrote about doing what is easy instead of what matters.
The counterargument is that life is unpredictable. I can choose everything optimally and still regret not living fully. There is no clear answer here. It depends on the person and their situation. It is subjective. But it is a concept worth staying aware of, because it guides decisions. Opportunity cost is real.