Besides diet, what are some of the reasons that would cause continuous weight gain ?

Multiple reasons: There could be internal abnormalities such as low thyroid function, low hormones (such as testosterone and dhea), there are some diseases such as cushing's disease and some other less common endocrine (gland and hormone related ) diseases that can cause unintended weight gain but high carb, processed sugar unhealthy diet and low thyroid function are probably the most common.