People should be able to want to be slim without someone saying it’s “gross and unhealthy”. If they want to diet and exercise to get the body they want then let them, why does it bother you what someone else does with their own body anyway?
Being healthy can mean being slim. I think generally people aren’t concerned with those who want to be slim, but those who want to be skinny to an unhealthy degree. I won’t speak for everyone, but I just want to see people living their life as healthy and happy as possible. That’s why it bothers me when someone doesn’t treat their body well.