Look up universe 25 and mouse utopia experiments. Exactly predicted this.
Comfort is also correlated to it, but I don't think it's exactly it either. A/C is very comfortable, and certainly there are psychological and physiological downsides to it. But I don't think there's any major rewarding experiences which A/C makes impossible or unthinkable. Maybe the taste of a cool glass of water after working all day in the hot sun? Some amount of resilience to high heat which makes traveling in hot places slightly more enjoyable? Experiencing the changing seasons more directly and consistently than just seeing a changing set of colors out your window? None of these are exactly trivial, but it doesn't seem like "optimizing for comfort removes all the color from life" is exactly right either.
Maybe it's just that suffering just for suffering's sake doesn't do anything. If you were in the universe where all houses are magically air conditioned 24/7 and offered to build a machine to make them uncomfortably hot for 4 months a year and uncomfortably cold for another 4 months a year, selling on the idea that, you know, variety is the spice of life, a little constant hardship toughens you up, I don't think anybody would buy that on its own.
But clearly people DO seem to go kind of crazy without pressure and challenges, whether you frame it as "convenience" or "comfort" or "eustress deficiency" or whatever. There's an entire category of hobbies and interests that are entirely based around doing uncomfortable or dangerous or stressful things (thrill-seeking). And even things like weightlifting, marathons, saunas, physically stressful things are often pleasurable and seem to obviously build character and resilience outside of just the act of lifting heavy things until it hurts too much to continue. I think weightlifting is a good analogy; hardly anyone would lift weights if there was no extrinsic reward at the end of it (e.g. health, vanity, bragging rights). The important thing isn't the inconvenience and uncomfortableness of lifting weights, it's something else.
Maybe "responsibility"? "self-actualization"? I think that's maybe closer to it.
The AC thing interests me as a topic–purely my opinion, but I do think that living in a very narrow temperature range changes your relationship with the outside world, and there's all sorts of good stuff to be done outside that's much harder to get motivated to do if the delta with what you're used to is too big.
As an anecdote: I mostly disliked hot weather in the past, but years ago when I got hooked on running, I found that I disliked the treadmill more than the heat, so I ran outside through a muggy NYC summer. It was miserable to get started but got a bit easier every year, and over time I've noticed, from the digital thermometer on my desk, that the temperature where I have to stop working and turn the AC on has slowly crept upwards. My running club puts on a 5k summer race series–if you'd asked me 10 years ago whether I'd want to run up a hill in the swampy Brooklyn heat every other Wednesday, I would have laughed out loud at the absurdity of the idea–but today it is genuinely something I love, and has been a highlight of my year for several years now.
All that to say, your point about suffering for suffering's sake stands: I cannot sleep without AC above a certain temperature/humidity, and I don't see any reason to force myself to suffer through restless sweaty nights. On the flip-side, though... I have never had more visceral, euphoric appreciation for the comforts of modern life than after a cold and drizzly backpacking trip where we spent the entire time damp and never really, fully warmed up for multiple days. 20-something years later I have the most vivid memory of getting in a hot shower when we got home.