Jungkook has always felt insecure and inadequate compared to the rest of the band. So, when his manager tells him to lose some weight, it's the last little push that sends him spiraling into a pool self destruction. He no longer sees himself as the golden maknae because you have to be perfect if you want to be golden, and he was the exact opposite of that. It had started off with just wanting to lose a couple pounds, but now Jungkook won't stop until he's perfect.