I have a couple of ways I explain it....usually I just tell them that in a nutshell whenever I'm around people I just have an irrational desire to get away from people, and it's hard to control sometimes.
But other times I explain it like this; imagine if you'd just heard some shocking news, i.e. a loved one had just passed away, and then you went to a dinner party. The emotional trauma would probably be on your mind all evening. The other guests may laugh and joke but you'll just smile along politely cause you don't really feel like laughing right now. Some guests may ask why you're so quiet this evening, but you might just hide it and shrug it off and just say you're tired or something.....and well, I feel like that pretty much all the time, like something terrible has happened, but if you asked me what was wrong, I could never actually point to any specific event. Everything in my life is fine, I just have this constant feeling like something terrible has happened.