We live in a constant state of heightened sensory receptivity (current research indicates the autistic brain has more neural sensory connections than the neurotypical brain), it's the reason why we tend to suffer from anxiety disorders as well.
When you live with the sensory volume "turned up to 11"
all the time, you may learn to "get used to" everything being too intense, and sort of intentionally zone out most of it, but it only takes a little push - like becoming emotionally upset - to send the brain into a state of overload that it just can't cope with. Then all those things that your brain has been working so hard to try to ignore can't be ignored anymore and suddenly you realize how LOUD everything really is.
I think that's why we function better when left alone. If there are no other people adding to the sensory 'noise' level, it's easier to cope with the too high volume of input we're already experiencing. When you start adding people and social cues to that, it's just too much to juggle.
This makes total sense and answers my question about this too. I was wondering why I do so much better when I am alone. Thanks Willard. You are always teaching me stuff here.