You could just have good genetics in which your body is resistant to weight gain or you have a low appetite to begin with. As shown by the worldwide obesity epidemic, this is apparently quite an uncommon problem. 75% of country overweight or obese.
This moral judgement whereby losing weight is "good" in absolute, is such bullshit. For most of history, humans have fought starvation literally every day, and often had to make do with minimal caloric intake for weeks or months - in that context, genetics that kept you thin were definitely very bad.