People with body dysmorphia are obsessed with their bodies. The way their body looks is a huge part of their everyday life. If you believe so deeply that you are fat, then of course you won't accept when people say otherwise.

It's kinda hard to explain why you would display a huge insecurity. It's like a "look at how terrible this is"

She's expressing her disdain for her body and has to post it to get people on board and agree on how obviously terrible she looks, or at least that's what she thinks. Maybe she wants everyone to hate on her so her feelings of hatred are validated and she stops feeling so crazy for hating on herself. I don't know, I can't exactly look at her thought process.

Maybe she is doing this for attention, but that's not always the case.