We have a health epidemic in America related to people being overweight or obese. Many will try to make changes to their lifestyle through highly restrictive diets - diets that they are not able to maintain for the rest of their lives. So, even if they lose weight in the near term, they eventually gain it all back and then some when they get off that extreme diet. This leads to mental health challenges about "failing again." This is what we refer to as yo-yo dieting.
The best diet then, is the healthy one we can maintain 90% of the time for the rest of our lives.