Maybe you lose weight because you moved, and the diet had no effect.

It was the diet, full stop. The diet was months after moving, and I stayed at the old weight before starting the diet, and I stopped losing weight when I went off the diet.

Or maybe your emotional state improved because you lost weight, and moving had no effect

I wouldn't have lost weight without the diet, which wouldn't have happened without moving. It's really the difference between A->C and A->B->C. The counterfactual where I successfully go on a keto diet without the social support of my sister isn't something that would have happened.

Everything happens for a reason, but most events have many complexly interacting causes, not a single unentangled one.

Let me be more specific in the counterfactuals I'm envisioning. You tell a person with a comfort eating problem to go on a specific diet, and they wind up saying they're on a diet but end up hiding their comfort eating. Any amount of diet advice is not going to work for them because dieting is solving the wrong problem. The right solution is teaching emotional awareness, alternative strategies for meeting your emotional needs, and general skills for improving your life.

I'll retract the "most" part of "most people got fat for a reason", because even if it's true I don't have the data to say that - all I really have is a story about how diets fail for people with certain issues.