Every fat person would become more physically attractive by slimming down.
Being more attractive leads to greater romantic success.
Stigmatizing excessive fatness encourages fat people to reach a more attractive body fat level; it pre-empts others from becoming excessively fat.
Suppressing fat shaming—through widespread “fat acceptance”—removes this social cue. That’s unhelpful.
When someone needs advice, a coward tells them what they want to hear; a true friend tells them what they need to hear.