Pietro notices that when there are empty seats on the bus, nobody ever sits beside a stranger. however, when the bus is crowded, people sit beside strangers all the time. he notices that the same thing happens in movie theaters, the school cafeteria, and even the classroom. pietro's observations suggest that people's behavior in these situations is governed by:

I believe the answer is: social categorization
social categorization refers to the process of anlyzing other people's characteristics in order to determine their position within the social group.

In the scenario above, strangers that meet on public property would most likely perceive other individuals as non-threatening outsiders.
So even if they necessarily have problem with them, they do not feel the need to get close to them either.

True. you know, tools like shovels.