Why do some people want to gain weight?

One of the girls I know thinks she’s too skinny and is trying to put on weight. She looks fine the way she is and doesn’t look that skinny. She said she is trying some food because she is trying to put on weight and doesn’t like being slender. I’ve dreamed of being slender again! Being heavy isn’t fun. I weigh 257lbs and 5’11” 3/4 but people think I look good and I need to weight more.