Last week I spent a few hours at Zoya's daycare to learn their tricks for putting her to sleep. I did learn a lot about that, but the most interesting things happened while playing with the other kids in her class.
There were three other children there with her, all between the ages of 1 and 3. At first they gave me a wide berth, since they only know me in the context of picking Zoya up or dropping her off. But then they warmed up to me (thanks for taking the lead, Julianna) and started to include me in their play. I was sitting on the floor, and they were crawling all over me, giving me books to read them and showing off their lego creations. I remember feeling a flash of concern, like something wasn't right. It didn't last very long, and I didn't think much of it at the time, but in retrospect it was clearly their disregard for personal space that momentarily threw me off.
Personal space is a learned social norm, and its very rarely violated. I'm not talking about being squeezed into a New York subway car - there's no expectation of space there. Unless someone's being aggressive or too flirty, most people go out of their way not to touch you or get too close unless it's appropriate. Which is why it stands out so much when three innocent kids jump all over you at once.