Excluding weight gain from corticosteroids or something exotic like elephantiasis, it is literally always overeating. I'm prone to it, and nobody can consider it an individual moral failing when obesity has become a massive global epidemic. But there's genuinely no getting around the fundamental reality that it's calories in, calories out. Apparently the majority of humans overeat when living a sedentary lifestyle with a readily available food surplus. The incredible success of GLP-1 agonists really hammers that point home if anyone was still unconvinced.