A while back, I attended a tutorial where we discussed a fictional patient case. In the introduction, the patient is described as having gained 45 lbs over the past three years. Most students seem to believe that this is a reasonable amount for someone to gain simply based on becoming more sedentary and adopting a worse eating style.
I strongly disagreed. I think that the vast majority of the time when someone has a stable weight for years and then suddenly gains a significant amount of weight (175 to 225 lbs, in this case) it's almost always going to have a medical cause. Our bodies cling to our setpoints quite voraciously. It's HARD to gain that much weight from a stable beginning. Of course, there are exceptions: chronic dieters, psychiatric illnesses such as depression, and individuals who happen to be unusual weight gainers or losers.
It took me a while to figure out why this was making me so angry. I think that this attitude that it's so easy to just gain 50 lbs if you 'let yourself go' is behind this idea that somebody who is thin is actively doing something right to maintain their weight. So if somebody is thin it follows that they are maintaining a 'good' lifestyle. If they adopted a 'bad' lifestyle, they would become fat. That is, naturally thin folks have a vested interest in believing that it is easy to gain weight because the conclusion drawn is that they have a good, virtuous lifestyle that has earned them the right to be thin.
Of course, if somebody unintentionally LOSES any weight (even 10 lbs lets say) we'd be all over a medical cause for it!