When we’ve invested our time, money, or emotion into something, it hurts us to let it go. This aversion to pain can distort our better judgment and cause us to make unwise investments. A sunk cost means that we can’t recover it, so it’s rational to disregard the cost when evaluating. For instance, if you’ve spent money on a meal but you only feel like eating half of it, it’s irrational to continue to stuff your face just because ‘you’ve already paid for it’; especially considering the fact that you’re wasting actual time doing so.
To regain objectivity, ask yourself: had I not already invested something, would I still do so now? What would I counsel a friend to do if they were in the same situation?