Psychologists tell us that huge rifts can devlope in relationships when one person changes and the other one does not.
When one person in as couple loses wieght that can cause a lot of problems because their relatioinship has often been built around--or evolved to be around--the weight of the two people--notice that most of the partners of the overwieght people are also overweight. Most of their activitries and interactions can revolve around food and in avoiding physical activities--choosing to watch tv for example and food prep and ingestion instead of engaging in sports or other physical activitirs.
So sometimes in a relationship where one person loses weight the other one may try to sabotage their partner. (Taking them to restaurants that only serve fatty foods for example. Or trying to insist they drink calorie-laden alcoholic drinks like they used to instead of letting them choose low-cal fruit juice or sodas etc.)
This is often subconscious of course--but there can be an overwhelming need to have the "old person"/old comfortable relationship back--and not have to try to adjust to the "new person" and new situation in which the one who lost weight now has more energy and isn't content to just sit around.
There is also the jealousy factor in which a man will now see his less heavy partner as being more attractive to other men than when she was overweight.
And the one who has changed can become quite unsympathetic to the one who has not changed. Was it Shay who was so critical of her husband for the fatty foods he was choosing? Showing the fat dripping off and wondering how anyone could eat that? She has changed her outlook and viewpoint and now is not patient with anyone who cannot change. (Religious converts often exhibit similar zeal.)
Psychologists often recommend couples counselling when one person changes a lot.
Oh also--women who go back to work or who start earning more money that their husbands also see this kind of situation develop.