I really like your point that there are goals and negative goals. I'd agree that the negative goal is harder to achieve, because you are aware every second that you are not doing something, or forcing yourself to do something that is about loss, rather than gain. Even the logic of work-out-to-lose-weight only has a "gain" in wearing something you have not been able to. In a similar vein a value-neutral goal is difficult; consider not weight maintenance, but the plateau when you lift, which means (for the serious) that they trade up their workout to emphasize something else.

Of course, there are biological limits to gain and no matter who you are the glands and endocrine system become less efficient with age, meaning that you will first plateau, and then fight against the downhill slide.