Being unchallenged can also give the illusion of having fantastic self control. My wife stays slender, in large part because she has no desire to eat to excess, and is sated by small quantities of sweet food. By contrast, staying thin same costs me a monumental effort. How can you possibly know whether you have superior self control or are simply lucky?
What is challenging is dealing with the finiteness of time. Having to prioritize my list of what I want to do/learn and slowly progress through it. Sometimes new things suddenly come up and move their way to a high priority. Other things will, at times, be moved down the list according to changes in my interest level or circumstances. But what is an entirely foreign experience to me is having something I want to do, arriving at the time to do it, and just... not doing it.
I can only think that people who have this kind of experience must be suffering from self-deception or dissonance between what they believe they want and what they really do want. Like the student who tries his damnedest on exam day but has not spent any time familiarizing himself with the course material. What we want is revealed by our behavior, not what we think. What I actually am is what I do.
As to the particular topic at hand, it is a core value of mine to keep my body as able as possible for as long as possible. My body is the vessel that conveys my existence through time-space and getting the most out of that existence requires optimizing my fitness. If I were to wake up one day twenty pounds overweight and pre-hypertensive, remedying that situation would shoot up my priority list. It would become an emergency situation that would simply have to be dealt with as expeditiously as possible. So when I hear people talk about how much they "really want" to lose weight but then present no behavioral evidence to support that statement, I can only conclude they they are either simply lying for the sake of garnering social approval or are deluding themselves as to their true desires.
I'm absolutely sure they're deluding themselves or lying. What i rationally want and what my lower systems want are fundamentally at odds. My inner passion for overeating threatens to overwhelm my logical mind every day.
I think when people say they want to lose weight, they mean that they wish they didn't have that extreme compulsion to eat that overwhelms their rational processes. Or that they could overcome it.
What bugs me is that people view it as a simple matter of self control, when it's actually about self control versus strength of desire. If your desires aren't that strong, it's easy to look like you have an iron will.