I think people end up having issues when they religiously follow diets despite everyone being different. My weight rarely changes and it is likely because what I eat is relatively consistent, and I am not trying to eat the recommended 2500 calories a day (for men). If I did I would probably be overweight. I also try to respond to the way my body feels, if I wake up and feel like my stomach is a bit bloated or sore (I have IBS), or if I don't feel hungry, then I will either only have a small breakfast, or weight and have a snack mid morning. Also 90% of the food I eat is made fresh, so I know its relatively nutritious.
Recently I had a family member who was talking about a diet they were on, basically everything was counted in sins, and you have so many sins a day, some foods however are sin free so you can eat as much as you like of those. So she would say she could have a a huge plate full of carrots or something as if they don't have calories in them. She was also saying its not the calories, but the type of food which you eat. She would basically forget about calories whilst on this diet as long as she follows the guidelines.
For me, I think 500 calories of chocolate, is going to sustain someone just as long as 500 calories of broccoli, so as long as you eat the right amount of calories, from anything, you will gain, lose or maintain your weight. The nutritional value of what you eat will probably have a impact on how healthy you body is, so things like how you smell, your skin, nails and hair, and also how healthy your digestion is, maybe how tired you feel.