Not everybody does; in fact it’s quite common to lose weight when highly stressed. Stress affects everyone differently and that includes how it may impact your eating habits and appetite. Added stress often leads to a cascade of unfavorable conditions like impatience, emotional fragility, bickering, and as you mention, weight gain. But weight gain is more due to food choices triggered more by mood swings, emotional desire for comfort foods, rather than normal body-appetite functions.
That said, from the physiological side, the primary stress hormone is cortisol which is secreted in excess during high levels of physical or psychological stress. Cortisol is important to the maintenance of blood sugar levels, blood pressure and will “over-stimulate” fat and carbohydrate metabolism for quick energy needs brought on by stress. And this is probably the main reason high levels of cortisol triggered by stress can increase and alter appetite, leading weight gain. Additionally, continuous disruption of normal cortisol secretions can also affect where fat is stored on the body. Some evidence points to people, with stress related high levels of cortisol, preferentially storing fat in the abdominal region as opposed to the hip areas when their calorie intake exceeds their daily calorie burn. Abdominal fat is more closely associated with cardiovascular disease than fat stored in the lower body.
If you are looking for help here the best thing you can do for both weight gain and stress is exercise. Exercise delivers “the triple benefit”: obviously it helps burn added calories you’re probably consuming, but the two less obvious rewards are exercise’s ability to naturally de-stress and help keep you guilt-free – just because you did it. I am not sure there is a better way to keep stress at bay than exercising. Additionally, exercise has been proven to be a better strategy for treating depression than any drug on the market. The hormonal changes, including the release of endorphins that take place during and following exercise, naturally soothe the entire body. And if you’re not exercising, this may be the best time to start.