Consider all the daily events that are really gatherings of people, social or not:
1) School classrooms
It doesn't get any more over stimulating that most classrooms. The K-6 classrooms are the worst for me, with their "neurological stimulation" of name tags, charts, maps, calendars, student art, and more. I felt overwhelmed as a student and I still feel overwhelmed when I enter these rooms as an educator or consultant.
Now, add the students. What was already an overwhelming space can become a nightmare. I've substituted in K-6 classrooms, which seemed to go well enough, but I was ready to collapse at the end of each day. Young children are... well... children. One or two is okay. Twenty? You can't possibly keep them organized, on-task, and clean. Part of teaching is knowing how and when to manage a classroom. The K-6 setting requires gently nudging most students. It's exhausting for a "normal" teacher.
The middle school / junior high, and high school settings are a little easier for me. I've taught English, math, photography, and journalism as a long-term substitute. The rooms are focused on single topics, which I like. But, the students are... teenagers.
Teenagers experiment with dowsing themselves in fragrances, various chemical concoctions, piercings, and so on. They are making statements. Their statements are difficult to decode for mere adults. They talk, constantly. They try to text until you confiscate a phone or two. It's a social atmosphere.
College and university classrooms range from 12-person seminars to 1000-person lectures. You can imagine the odors, the noises, the chatting during lectures, the eating of nacho chips, and so on. College students range from the fragrance-soaked to the non-bathers. Then you have the students who cannot figure out that how they dress conveys a persona. Even as a teacher, I've wanted to tell students that shirts should fit, pants should not fall down, and poor hygiene might explain the empty seats around them.
Offices are like classrooms, but with a lot less order and far more noise. Depending on the arrangement of cubicles, open space, or walled offices, the smells and noises can range from mildly annoying to completely debilitating.
Workplaces are odd social hives. Studies show the average office worker has four to five productive hours per day (seriously, that's it). While waiting for computers, printers, and on-hold with vendors or clients, people in offices chat. They chat a lot. They'll approach for no reason and talk about things about which I could never feign interest.
The people smile and chat about sports, politics, their churches, the latest movies, and so on. There is seldom one conversation; more often it is four, five, or even a dozen conversations. And I'm compelled to try to follow voices and to untangle them so I can figure out what is or isn't important to me. Trying to focus on one person talking to me is frustrating and exhausting.
I want a silent little office and a list of tasks I should complete. Send me e-mail, don't waste my time telling me about Little League games or church events. Please, leave me alone! Go away!
I'm not interesting in being friends with most coworkers. I don't want to learn the office politics. I only want to do my job. Social clubs are for other people, I don't need work to be a club.
The too-loud phone talker, the over-colonged salesman, the woman who chews gum while talking, the man with new baby pictures, the "helpful" gossip who stops by every desk, the nosey tech, the friendly manager. Please, let me work. I don't understand you and don't want to have to keep trying.
3) Shopping almost anywhere
The stress depends on the store type and the customers. Some stores are worse than others. I like the garden shops at home improvement stores. I hate the chemical aisles. I love grocery store bakeries, but I hate to be near some of the "pre-prepared" deli counters that stink of oil and deep fried everything.
The people in some stores keep a nice distance and move politely. The clerks are not pushy, and I can relax.
Then, there is the Apple Store. I love my Macs, but I hate the store on weekends or busy weeknights. When I need something, I go during the day, mid-week. I know success is great, but do I need to share 800 square feet with 100 people, displays, and service counters? It's too much for me. I've walked out more than once to return later.
4) Public transit
I stopped using it. I cannot handle the people. A single bus or train ride can render me useless for two or three days.
5) Government offices
The lines at the DMV, county clerk, et cetera are like public transit.
I like the more formal places, because fast-food dining is loud. I can't bear the sounds of deep fryer alarms, microwaves, and other devices. Every fast-food chain is overwhelming inside, so I prefer take-out. Ideally, a drive-through or delivery.
I don't want to deal with children running about the dining area. I don't like the filth most people leave behind, when it isn't that difficult to bus a try to the trash. I just can't stand the muck of most fast-food dining areas.
Semi-formal is much, much better. Most of the time. Then, there is Big Bowl, which has wonderful Asian-influenced meals but imagines itself to be a nightclub. They play the music too loud, causing the diners to speak louder yet. I hate it, but I love the food. During the summer, you can eat outside, but the outside of a mall or strip center often has its own speakers playing loud music, car exhaust, and random groups of loud people walking past the tables.
I also find Don Pablos, a Tex-Mex chain, too loud. The customers get louder and louder. Add in alcohol, and the customers can be especially annoying. They need to turn down the music, too. I prefer smaller, family-owned Mexican places. They tend to be quieter, less crowded, and more attentive.
All three of my favorite places are semi-quiet, dimly lit places with good food.
Unfortunately, I've even had to leave my favorite restaurants because of noise or odors. Too many people is just too much for me, especially if the people are loud. You won't find me at a bar watching Monday Night Football or the Super Bowl. Sorry, just not able to deal with it.