[mdomans]

They usually have underlying psychological condition which they fail to address. Note that all the "regain" studies were done in cohorts that had either severe problems or lacked proper professional assistance (which these days you can get online for 100USD).

The fact that you can loose weight is simple physics. The fact that these people often have endocrine issues is near tautology because adipose tissue HAS endocrine activity and thus ones changes hormonal profile if in excess. Usual set of changes include: insulin resistance, lower testosterone, low thyroid hormones, leptin resistance, higher SHBG and usually higher estrogen.

Doesn't mean dieting won't work but if someone is long-term overweight he needs more work than just a difference in bodyweight.

[oceanghost]

For me, it was celiac disease.

[mdomans]

It happens. There are multiple others possible problems, but I'd say that it's 20% other and 80% diet