Autism

The thing is the degree to which you have to mask. Human communication is full of aspects that are interpreted, performed and learned in an instinctive way, such as how often you should be looking at someone in the eyes, for how long, which expression should you have while doing so, and so on, just to pick a very narrow category.

Because the brains of people in the spectrum have some differences, the "correct" instinctive way of doing those things is almost usually different, some people whose brains think that isn't the correct way of performing non-verbal communication will react negatively, sometimes without being capable of explaining why, and will instead retort to vague attacks such as "XYZ is such a weirdo/gives me bad vibes/is a creep", even when they aren't really doing anything wrong.

You may have heard or read of the "doesn't make eye-to-eye contact" pointer to diagnose autism. That is true of a subset of autistic people, there are other autistic people whose natural prefered amount of eye contact is different than the norm, and there are plenty who perform the socially acceptable amount of eye contact because they're masking, even though it feels unnatural, or feels annoying, or forced, or is even energy-consuming. Now consider the same for voice tone, sarcasm, gender roles in communication, hand gestures (depending on the country), how much it is socially acceptable to discuss your interests specifically, and so on, and so on, which are often also rules that aren't laid out with precision, because NTs don't need them to be laid out with precision. There's a dual problem in that you're both constantly forcing yourself to perform in a way that feels unnatural, and that you have to consciously seek out for signs that they're being interpreted the correct way, because otherwise NTs aren't going to tell you, and all of that is taking energy out of you.

As personal anecdotes. I didn't know how to mask in my early teens, and consequently, I was often left out at best, when not mocked or ridiculed, in physical social settings, while online I had no issues whatsoever to make friends even though I was annoyingly smug (people will be far more willing to overlook your bad traits if you're capable of correctly performing that you're part of the in-group). Nowadays, if I'm in a place far away from my birth region, I don't have to mask too much because people will usually assume that my "weird" traits are just a cultural thing.