Judging people is a normal human thing--it's really hard not to do, and it would be stupid not to do it at least some of the time. Say when a person is running toward you with a gun, judging whether it's a law enforcement officer chasing the other person who ran that-a-way or a predator threatening you with injury or death is a very valuable judgment to make.
Some people have very clear rules for judging people in a way that's dismissive: are you a good person or a bad person? If you're a bad person, then I am allowed to other you, to treat you with disrespect and contempt, to feel entitled to abuse you. I get to feel a little better because surely I am not as bad as you. Lots of people used to use religion, because most religions have clearly stated rules you can use to judge, and designated punishments for being bad, and often also a rule about avoiding bad company or even shunning people judged to be bad. Then there was the gentleman's code; basic manners have sometimes been perverted into a test, a straightjacket for behavior and a way to exclude.
Because that's what all of this is about: excluding some people from our tribe, from the rights of an equal, from the respect and consideration we'd want ourselves. These are almost never rules people apply only to themselves, or even to themselves at all. They only use them against others. They only use them to pick somebody, a person or a group of people, who they can then target for unfair treatment, ridicule, bullying. And they can do all that, they can act evilly toward those people, without any guilt or sadness or other negative consequences, because they first qualified them into the "other" category and out of the "just like me" category. They judge the othered people not worthy of the treatment given to acceptable people like themselves.
Right now politics is a popular topic for this test, but I don't believe anything crosses traditional boundaries of politics, religion, or identity the way people's body weight does. It's being masked as health, and justified by concern trolls through a claim that fat people's medical issues cost them money (in increased insurance premiums and other ways), but it's still fat hatred. You can tell because there's no similar concern about people who are unheathily thin, or have treatable high blood pressure, or asthma, or any other health concern that requires routine medical treatment or might lead to an expensive medical intervention later in their lives. Meloukhia
points out some of the effects of this pretended concern about health. One is nosiness.
[T]hey feel quite comfortable quizzing other people about personal medical issues, and offering unsolicited advice on treatments or lifestyle. They also feel entitled to judge the activities of the people around them, even when those activities have no actual impact on their lives. And even when people are unhealthy, aware of it, and perfectly okay with that fact, with no personal diminished quality of life. A fat person eating a doughnut in Cleveland and deeply enjoying it has absolutely no material effect on my existence, just as an asthmatic who doesn’t adhere to a care plan in Miami doesn’t influence my life in any way.
What about choice and agency? Don't you get to make choices about your own life even if others don't like them? I had children and a lot of people think adding to the population is wrong, but there's no law against it yet. Apparently the health concern trolls feel free to judge every decision you make, because you're supposed to be choosing only based on health. Not pleasure, or expense, or lack of time, but only health. And not reality, not individual decisions based on your doctor's advice and your personal circumstances, but the received wisdom about what's best for others.
It’s less about how people feel—Are they happy? Are they stressed? Are they unhappy? Do they want to be healthier?—and more about how other people perceive them, as ‘unhealthy.’
Meloukhia makes other good points about backfiring and stigma; worth reading it all.