You're right, there are certainly many mental factors that contribute to weight gain, such as hunger, willpower etc. that vary from person to person. However, a

You're right, there are certainly many mental factors that contribute to weight gain, such as hunger, willpower etc. that vary from person to person. However, as far as actual physical weight gain (controlling for height and muscle mass), the dominating factor is calories in vs. calories out. For people like you who are tall and skinny and it seems impossible to gain weight, you are either not eating as much as you think you are consistently (i.e. not correctly tracking your calorie intake) or you are grossly underestimating how many calories obese people consume on a daily basis. Just eating at McDonalds 1-2 times a day doesn't mean anything if you are just getting a burger, but if you get three burgers, a large fry and large coke on a daily basis, that is going to make a difference. Along with mental factors, poor dietary upbringing, as well as horrible (and consistently changing) nutrition advice are strong contributors to problems with weight gain.