“Unhappy memories are persistent. They're specific, and it's the details that refuse to leave us alone. Though a happy memory may stay with you just as long as one that makes you miserable, what you remember softens over time. What you recall is simply that you were happy, not necessarily the individual moments that brought about your joy.

But the memory of something painful does just the opposite. It retains its original shape, all bony fingers and pointy elbows. Every time it returns, you get a quick poke in the eye or jab in the stomach. The memory of being unhappy has the power to hurt us long after the fact. We feel the injury anew each and every time we think of it.”