I like Gary Hayden’s articles on, I suppose, life, in Mind your Body. About two weeks ago, he wrote something on why we often regret more the things we did not do in our lives.
We regret more the things we did not do because we are free to imagine what might have been. We torture ourselves with what almost certainly would not have happened even if we had done things differently.
So imagine: you are unhappy with your marriage because the person you married is, a, b and c. You are also unhappy because there was that one person that got away. Here, you can let your imagination run wild and fantasize the perfect marriage that could have been but never happened. This causes more regret than reasons a, b and c of your unhappy marriage.