I don’t think you can convince someone to lose weight, but I do think you can threaten them with loss.
The coach who threatens to cut a student-athlete if he doesn’t get in shape.
The doctor who threatens to stop seeing a patient if he doesn’t lose weight.
The friend who courageously points out that it might be easier for you to find a partner if you were at a healthy body weight. After all, humans are more attracted to people with a healthy BMI.
Indeed, change most often happens by going deeper into the pain, not avoiding it.