Agreed that behavioral changes can certainly help people maintain weight, but, given that adipose (fat) cells appear to recruit stem cells into additional adipose cells (or whatever's going on such that the obese have both more and larger adipose cells). I'd expect that this change in adipose mass creates a hormonal disruption, making weight loss a difficult lift through behavior change.

Adipose cells secrete hormones, notably leptin and adiponectin. I wouldn't be at all surprised to find that major changes in adipose mass disrupts this balance, possibly past a tipping point.

Not a bio guy, just an area of interest - it's beyond fascinating. They're still identifying hormones secreted by adipocytes. See https://cen.acs.org/biological-chemistry/biochemistry/Hormones-reveal-secret-life-fat/96/i40