User blog comment:Mgdodl/"The Gamblers" Discussion/@comment-3045936-20200131042502/@comment-3169324-20200201022525

If we think of luck as the probability for the positive outcome of a given event, then someone with average luck would have a 50/50 chance of getting a positive outcome. However, the Winchesters have been getting positive outcomes for decades in almost every situation, so now that their luck is at a normal level, they need a string of bad luck events to regress toward the mean.

Let's say they got a positive outcome in 100% of all situations where their luck applied, for the sake of simplicity. Dean is currently 41 years old. Assuming he lives as long as the average male in the USA, he has another 35 years to live. This means that he will need to get more negative outcomes per day than he did during the previous 41 years, for the rest of his life, to get to the 50/50 average (and even then it'll be close). Naturally, this results in bad luck streaks and resembles a curse.

Now, statistics are obviously more complicated than this IRL, but since this is a supernatural concept in a TV show, my guess is it'd work similarly to how most people think statistics work (for example, that the odds of getting heads or tails on a coin are influenced by the flips that came before it, or that the expected outcome of something is the only possible outcome).