I think I know why, but I am not 100% sure.
Most Helpful Girl
Maybe because they feel the person they're sharing their life with, will understand them and how they feel. Being overweight is not the easiest, and when you constantly struggle on a daily basis, (whether you're trying to lose weight or not) your confidence gets shattered daily, you may seem happy to the outside world but underneath there are a lot of issues, personally speaking anyway, so maybe they need someone who just understands... this may be the case, I've lost most of my weight now but still have a long way to go, I've been fortunate enough that the guys I've dated have been in pretty good shape, but that's just luck I guess lol
Hope this helps0