Sunk Costs - An Economic Concept for Everyday Life

In five years of college, I learned about a wide variety of topics including programming, digital design, microelectronics, and engineering methods. However, there is only one concept that I attempt to use in everyday life, Sunk Costs. A formal definition of sunk costs from the Economist.com states:

When what is done cannot be undone. Sunk costs are costs that have been incurred and cannot be reversed

The example used in class to help us conceptualize involves grocery store lines. Say you're in Wal-Mart and selecting a check out line. The only two lines open seem to be about the same length so you pick the one that you think is best.

Ten minutes later, you've barely moved and the other line seems to be moving at about twice the pace. Most people will inexplicably stay in their current line, because of the time they've already put in to waiting there. With consideration for sunk costs, you should switch lines.

Why? The time you put in has already passed. You can't get it back regardless of your course of action. If the other line will get you out of the store faster, you should switch lines...always.

In case I didn't explain that well, consider this math breakdown:
x: amount of time you've already waited in line
y: projected amount of time you will still have to wait in current line
z: project amount of time you will have to wait in the other line

Total time in current line (if you stay) will be x+y
Total time if you switch lines will be x+z
Since you think z < y, it's a no-brainer to switch

So how and why do I apply this concept to everyday life (outside of the grocery store)? Anytime I see a better path than the one I'm on, I take it. No questions and no hesitation. If at anytime the new path turns worse than another (including the old one) I switch.

I apply this to everything I can. If I'm reading an article and it's boring, I quit reading it. If I rented a movie, but an opportunity comes along to hang out with friends instead, I ignore that I rented the movie. If I'm writing a cover letter for a job and see another job that I'm more likely to get or will give me more satisfaction, I'll write that cover letter instead. You can always go back to the article, movie, or cover letter later on, but you may miss out on the better opportunity if you don't take it when it arrives.

I'll admit that the rented movie is a bad example because at some time, the rental expires and it would cost you more money to rent it again. But I would easily trade that $1-3 for a fun night with good friends.