for example, you don't have to do anything in particular to gain weitht, i.e. eating any specific foods. All you might ultimately have to do is just like eat normally and excercize/live your life normally, and in doing so you could gain weight. But even if you did not gain weight you would not lose weight, either unless you altered what you ate/how you excercized. In essence, you were not even trying to gain weight, and yet you did anyway, without even trying. When it comes to LOSING weight, however, you actually have to do certain things, i.e. eating certain foods which are almost always impossible to come by and expensive, and excercizing regularily to even lose any weight at all. And in order to maintain your weight loss you have to continue to eat the expensive foods and excercize like that, like you can't just eat normal inexpensive foods and live your life normally or else you will gain the weight back and/or you will not lose any more.