Most people use collocations all of the time, but they don't realize that is what they are called. A collocation is a grouping together of words in a certain order to make a common expression. You find that certain words tend to pair up with one another. For example, "mortal" and "combat" are often paired or "crystal" and "clear". Now, if you think about "thin" and the words that it are frequently collocated with it, you may come up with "thin and beautiful" or "thin and attractive". Conversely, we find that "fat" is often collocated with "ugly" or "disgusting".
These collocations aren't merely expressions, but part of our collective consciousness about how we regard weight and appearance. The truth is, however, that weight doesn't always have such a clear cut relationship to appearance. There are plenty of thin people who are ugly and fat ones who are beautiful. Beauty is considered to be largely subjective, but, in reality, it is a measure of certain types of symmetry which almost all humans find more appealing. Being overweight is seen as "ugly" mainly because the weight changes the symmetry or favorable ratios that are connected to beauty and because affluent cultures place a higher status on being thin.
The point of this post isn't to argue about "big" being potentially "beautiful". My point is that the collocation about "thin" being beautiful may actually play a part in the psychology of people regaining weight. Many women believe that they will be beautiful once they lose weight, but the truth is that many women lose weight and merely look average or even below average. The basic structure of beauty can't be acquired through weight loss. If your facial symmetry or waist to hip ratio at a lower weight are not appealing, you won't be beautiful when you are thin. You will just be thin.
Sometimes I wonder if the expectation that one will be regarded much more positively or become "hot" by dropping excess weight sets one up for a major fall when they get closer to their goal. What if you find out as you approach your goal that you aren't some great beauty hiding under all of that weight as everyone seems to be telling you when you're fat? What if you find that you are almost as invisible to the wandering eyes of men seeking partners as before or that you don't look really great in your smaller size clothes? It happens. In fact, it happens more than people realize as we never view ourselves as others do. People sometimes think they look great because they lost weight, but they actually look quite average.
One thing I've noticed when I view the progress pictures of the brave people who put up pictures of themselves after weight loss (and they are so much braver than me) is that many of them look healthier (and therefore "better"), but not necessarily "beautiful" or even "pretty". I had a similar experience when I lost weight in college. I don't think I looked beautiful or pretty, and it seemed that my nose really looked bigger with a smaller face.
So, what happens when a woman loses a lot of weight and she doesn't become beautiful? Well, the expected "reward" of beauty doesn't come at the end of a lot of effort and the chances that one will regain are increased. I'm not saying that is all it will take to set off the cycle of gaining weight after losing, but that it very well could be a factor. For this and many reasons, I think that overall fitness, mobility, and health should be the main focus of weight loss rather than appearance. Without having this priority in place, there is an increased probability of eventually looking in the mirror and saying, "why bother", when confronted with the choice of overindulging in food.