Why do some people think it's okay to hate and insult naturally slender people?

I'm not a naturally slender person but that upsets me a lot. I mean we're are all meant to be different and we're beautiful the way we are. My best friend is naturally slender and she gets a lot of hostile comments and people say she's too skinny and thick is in these days and she needs to put on some weight. She even said she wishes she has affect the body like me and my body is body goals. I thought body goals to be healthy?