It's because people making strides to become human makes them feel bad about themselves. It reminds them that, deep down, it's still considered shameful to be fat, no matter what HAES and FA try to push.  It reminds them that, no matter how much they tell themselves "I SLAY" in the mirror, they will always feel the subtle, visceral disgust of looking at hundreds of pounds of fat. That visceral disgust is why they feel that they need to pull others down with them, since "if everyone's fat, no one's fat".  It's also the reason why they have to scream at the world how happy they are, kind of like the person in a shitty situation saying "it's not all that bad".  When someone decides to lose the fat, it's saying "this way of life is not good enough for me, I deserve better", which is unacceptable to a fatty who's built an emotionally fragile house of cards, all based on the lie of "I can be fat and happy". It seems only fats experience this though, since I only experience a brief moment of "well that's a damn shame" and a shoulder shrug when I see fit-to-fat transformations.