Why are some people miserable when they were thin?

I KILL to be thin again!!!! Are people crazy? I was tall and thin until puberty. BOOM! I’ve gained weight and got fatter. One of my cousins said she used to be thin, had six pack and in shape. She said the smallest she was is a size 4 and 120 and she said she was miserable. She said being thin made her miserable. She kept turning heads, got hit on, got pressured to look good but she wanted to be herself. She was thin all her life and now she gained a lot of weight she’s happier. She did lose a lot of weight after having two kids but she just let herself go after being busy with kids, work and she gained 40 points and she a lot happier. She said, “I’m happier at a size 12 than I was at 4. Not everyone wants to be thin and not everyone is happy being thin. If you wanna be slender fine but being healthy is more important and besides, men love women with a bit more weight on them. Men don’t like thin women sweetie.”
I would love to be thin again and have six pack well maybe not that but a toned stomach.
Why are almost being depressed when they’re thin? There’s so many people that KILL be be thin! What is wrong with people? Not saying that big is bad.