tl;dr: people like others have their vices because it's a comfort to them having their own vices.
It's a great point about others wanting you to be held back, these people tend to be the most bitter, jealous, cynical and close minded people around. So I wouldn't say it's most people, but everyone who's improving will notice these people pop up with their mind games (even if subconscious mind games).
Your theory on worrying about looking bad is probably true for a small amount of people, I don't think they consciously go "well if he has hair and has shown some progress in his world, what am I doing with my life? Can't let it happen".
Instead for the majority of these people it's clearly that they like the comfort of having someone like them, then their weaknesses are more understandable. They don't like seeing someone successfully eating no sugar or crap, because they like their chocolate and crisps and fizzy drinks, they'll offer these to other people, it's comforting to know they aren't the only weak ones.
It feels like I'm mentioning my new gym regime in half my posts recently so sorry if anyone's sick of it, but I'm new to this and you notice a lot of things you've never thought about before. This exact topic is one of them, the genuinely nice people in my life are generally proud of me for going out to the gym a lot, they encourage it, even some of them with issues of having weight, and some who have openly shown they have tried and failed at losing weight, still are really happy that I'm doing it. They ask me about it all the time and take an active interest, which is good for my own motivation. These are of course also the people I naturally keep close to me, and also when it comes to baldness, they may have at some point shown the same f*****g annoying attitudes we expect from society (thinking hair loss is a funny joke, thinking "it's just hair", literally a friend said a month ago about Conte who had a hair transplant "He should've owned up to it like a man" ugh) but that's not their fault. Many of us on here acknowledge that if we had perfect NW0's we'd be just as blissfully ignorant and have these attitudes ourselves, it's hard to HATE people for it, but with my close and caring friends they at least acknowledged at some point down the line that I'm sensitive towards it, and act accordingly. You can't really ask for much more than that..
And the other side? Never mention my gym routine or even somewhat suggest it's a waste of time. I don't go out of my way to mention I go to the gym but it's a relevant enough part of my life now, for example I now go the other direction out of work to go there, so it's inevitable I mention this to people as I have to go that way for the gym now. I've noticed the most bitter people will move the topic quickly on, they won't even dare to acknowledge it, it hurts them.
It's not that these people are all inherently bad in all ways, one of them is a good friend of mine at work and he's very fleeting towards "gym bunnies" as he calls them, it's clearly not for him and that's fine if he has no interest, I don't think he actively wants to put down my efforts, but in his worst moods he will take un-necessary attacks for example- implying people who go to the gym stink. This is from a guy who regularly didn't smell great on a daily basis! Most people who go to the gym shower twice a day, smell better than anyone, if he was showering every day then he sure as hell wasn't washing his clothes right.
But continuing with that point that not all of these people are inherently bad, my brother is definitely not a bad person at all, and he's on the surface somewhat supportive and in many aspects of my life tries to make sure I'm improving and he can help whenever he can. However I get the feeling after his initial encouragement, now that I'm sticking with the gym and my physique is improving, he's just a little jealous, he still asks about it at least, but doesn't listen at all when I go into my program etc. and I don't even blabber on about it. I could be wrong here, but I get this sense that as he has no time for the gym, or maybe in his mid-30's he's starting to feel that aspect of his life is passing him for good (with regards to weight lifting, not general cardio fitness which I'd say he'll go back to).
As this post is ridiculously long anyway I may as well throw out a related story to just how bitter and cruel people are capable of intentionally being. I knew these student girls who house shared together, were friends before that, one of them lost a good bit of weight over the summer before coming back for uni, and it seemed she wanted to lose more however- she was doing it in secret. Weird isn't it? She didn't acknowledge she was on a diet, she'd have just tonic water and pretend it had vodka or lemonade in it, she was going for a run stupidly early at like 6am and literally sneaking back in the house "being quiet as I didn't want to wake anyone" and not acknowledging she was ever going for runs, everyone found this really weird.
Now the worst part; she intentionally started giving out food to the rest of them, buying cakes at times literally every day, chocolate bars, crisps, "little treats" for her friends. It even got more aggressive that it was practically peddling sugar "Oh go on spoil yourself!"
This went on for a good few months, the rest got suspicious, told me that it's odd this keeps happening but it is such a mental thing that you can't believe your friend would actually be doing it. But it became clear she wasn't eating the food she was handing out, she'd even cut a piece of cake for herself and disappear, it was after a few months that one of them went to the extreme step of looking- and finding, a piece of cake thrown in the bin. She actually asked about it in like a casual way, not accusatory or anything and the response "oh I've had a weird tummy all day so I changed my mind" but I was there plenty of times when she'd cut a piece for herself, maybe take just 1 fork of it, then like pretend to get lost in her phone, walk out of the room with it, etc.
They started openly joking about her friend making them fat, and that was awkward but it was becoming more clear that this was the kind of psychopathic behaviour that was actually going on. I mean to me that's such a horrible thing, losing weight and looking better, and becoming obsessed with her own friends looking worse, and it was an obsession, they reckoned she must've been spending a tenner a day on crap food, that's over £70 a week, as a student.
So anyway they eventually confronted her, coming up to Christmas, she lost her sh*t, she didn't get angry or anything she just got real nervous and shaky, wouldn't admit to it and said she's not going to be "charitable" any more. It was coming up to Christmas and she left unexpectedly, and didn't come back after break. Her stuff was gone when they came back and she just let the landlord know, none of her friends.
Nuts. That's the extremities that people can go to, in relation to your example of how people like to hold others back.