Many of the secrets on PostSecret are sad and this one is no different. Here we have someone who seems to love someone else wanting that someone else to lose weight so that his/her family will love them as much. If these family members don't love the person with their extra weight, who says they will love them without it? And can the author of the postcard really love this person if they want them to do something shallow to gain someone else's love?
It seems more likely to me that the author wants the person to lose weight and is using his/her family as an excuse which is sad. Helping and encouraging someone to lose weight for their health is a good thing. Doing the same because you find yourself loving someone less for their weight is just sad.