It is said that obesity is related to many factors, but why? I will quote an interesting description that is relevant to my blog.
  

The various theories fight among themselves, as if they are all mutually exclusive and there is only one true cause of obesity.
For example, recent trials that compare a low-calorie to a low-carbohydrate diet assume that if one is correct, the other is not. Most obesity research is conducted in this manner.

This approach is wrong, since these theories all contain some element of truth.

THE MULTIFACTORIAL NATURE of obesity is the crucial missing link. There is no one single cause of obesity.

What we need is a frame work, a structure, a coherent theory to understand how all its factors fit together. Too often, our current model of obesity assumes that there is only one single true cause, and that all others are pretenders to the throne. Endless debates ensue.

Too many calories cause obesity. No, too many carbohydrates. No,
too much saturated fat. No, too much red meat. No,
too much processed foods. No, too much high fat dairy. No,
too much wheat. No, too much sugar. No,
too much highly palatable foods.No, too much eating out. No
   

It goes on and on. They are all partially correct.  *snip*

All diets work because they all address a different aspect of the disease. But none of them work for very long, because none of them address the totality of the disease.
Without understanding the multifactorial nature of obesity-which is critical - we are doomed to an endless cycle of blame. [1]“

   
How the author views weight gain is great, because there is almost no one who can think in such a multifaceted way. 

However, what I want to say is that obesity is not a multifactorial disease, which means that obesity can be complicated and intertwined with factors and theories we can see, and cannot be completely explained, but when we focus on the unseen workings of the intestines, the cause of weight gain would be mostly specific.
   

References：
[1] (Dr. Jason Fung, The Obesity Code, 2016, Pages70, 216-217）

Most of the Intervention studies on obesity so far have only been comparative studies that involve reducing the intake of calories, regulating the amount of carbs or fat, or increasing exercise, etc. The experiment  they are doing is also in the range of (B). 

However, that is not a fundamental way to deal with being overweight, as Dr.Jason Fung mentioned, so regaining weight (the rebound effect) is inevitable if you eat as before.

▽In contrast, when (A) the base weight value goes up by intestinal starvation, there are various factors related.

For instance, it is said that the following affects weight gain:
・Skipping breakfast  ・Late dinner　・How many meals you eat
・Refined carbohydrates　・Processed food　
・Lack of fiber intake  ・Unbalanced diets

These are some of factors that are related to part (A) of the graph.

The important thing here is that each of these factors seems to be linked to weight gain, but there is no causality between each factor and outcome. Rather, they are related to inducing intestinal starvation. (In this case, some may say intestinal starvation can be “confounding factors”).

As I’ve already explained in another article, a combination of some of these factors below (from  category 1 to 4 of the table) happening simultaneously or overlapping, can cause intestinal starvation, and the occurrence of intestinal starvation may be pinpointed in the unseen workings of the entire intestinal track (or it may be the small intestine only).

[Related article]　Three (+one) Factors to Accelerate “Intestinal Starvation”

Many theories that are believed to make us fat fight among themselves, as if there is only one true cause of obesity, as Dr. Jason Fung mentioned.
If you ask me, each of them is not the direct cause of being overweight. Most of them are just the factors that are related to inducing intestinal starvation. 

The root cause of being overweight is the increased base weight (value), which is caused by intestinal starvation. So, I believe the causes of weight gain would be mostly specific in the unseen workings of the intestines.