What does it mean that opportunity cost thinking?

Every choice to do X is a choice not to do Y — consider what you give up.

Example: You decide to spend Saturday rewriting a side project in a new framework. That's 8 hours you didn't spend writing, exercising, learning a new domain, or resting. The rewrite might be the right call — but only if you've actually weighed it against what those 8 hours could have produced elsewhere. Most people never run that comparison. They evaluate the thing they're choosing in isolation, as if the hours came from nowhere.

Try this: Pick one commitment you made this week — a meeting you accepted, a project you started, a purchase you made. Write down three specific things that time or money could have gone toward instead. Now honestly assess: did you consider any of those alternatives before committing? If not, you've just caught opportunity cost neglect in action. Repeat this weekly until the comparison becomes automatic.